Create path categories for metadata folders (#1718)

## Description

Corso needs to store some transient information between backups so that it can perform more efficient incremental backups. This transient information will live at a path `tenant-id/service/user-id/categoryMetadata`. This patch adds an initializer, category enums, and tests for these new path locations

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor

## Issue(s)

* #1685

## Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/pkg/path/path.go

@@ -250,7 +250,7 @@ func (pb Builder) join(start, end int) string {
 	return join(pb.elements[start:end])
 }
 
-func (pb Builder) verifyPrefix(tenant, resourceOwner string) error {
+func verifyInputValues(tenant, resourceOwner string) error {
 	if len(tenant) == 0 {
 		return errors.Wrap(errMissingSegment, "tenant")
 	}
@@ -259,6 +259,14 @@ func (pb Builder) verifyPrefix(tenant, resourceOwner string) error {
 		return errors.Wrap(errMissingSegment, "resourceOwner")
 	}
 
+	return nil
+}
+
+func (pb Builder) verifyPrefix(tenant, resourceOwner string) error {
+	if err := verifyInputValues(tenant, resourceOwner); err != nil {
+		return err
+	}
+
 	if len(pb.elements) == 0 {
 		return errors.New("missing path beyond prefix")
 	}
@@ -273,6 +281,48 @@ func (pb Builder) withPrefix(elements ...string) *Builder {
 	return res
 }
 
+func (pb Builder) ToServiceCategoryMetadataPath(
+	tenant, user string,
+	service ServiceType,
+	category CategoryType,
+	isItem bool,
+) (Path, error) {
+	if err := validateServiceAndCategory(service, category); err != nil {
+		return nil, err
+	}
+
+	if err := verifyInputValues(tenant, user); err != nil {
+		return nil, err
+	}
+
+	if isItem && len(pb.elements) == 0 {
+		return nil, errors.New("missing path beyond prefix")
+	}
+
+	metadataService := UnknownService
+
+	switch service {
+	case ExchangeService:
+		metadataService = ExchangeMetadataService
+	case OneDriveService:
+		metadataService = OneDriveMetadataService
+	case SharePointService:
+		metadataService = SharePointMetadataService
+	}
+
+	return &dataLayerResourcePath{
+		Builder: *pb.withPrefix(
+			tenant,
+			metadataService.String(),
+			user,
+			category.String(),
+		),
+		service:  metadataService,
+		category: category,
+		hasItem:  isItem,
+	}, nil
+}
+
 func (pb Builder) ToDataLayerExchangePathForCategory(
 	tenant, user string,
 	category CategoryType,

src/pkg/path/resource_path.go

@@ -6,14 +6,27 @@ import (
 
 var ErrorUnknownService = errors.New("unknown service string")
 
+// ServiceType denotes what service the path corresponds to. Metadata services
+// are also included though they are only used for paths that house metadata for
+// Corso backups.
+//
+// Metadata services are not considered valid service types for resource paths
+// though they can be used for metadata paths.
+//
+// The order of the enums below can be changed, but the string representation of
+// each enum must remain the same or migration code needs to be added to handle
+// changes to the string format.
 type ServiceType int
 
 //go:generate stringer -type=ServiceType -linecomment
 const (
-	UnknownService    ServiceType = iota
-	ExchangeService               // exchange
-	OneDriveService               // onedrive
-	SharePointService             // sharepoint
+	UnknownService            ServiceType = iota
+	ExchangeService                       // exchange
+	OneDriveService                       // onedrive
+	SharePointService                     // sharepoint
+	ExchangeMetadataService               // exchangeMetadata
+	OneDriveMetadataService               // onedriveMetadata
+	SharePointMetadataService             // sharepointMetadata
 )
 
 func toServiceType(service string) ServiceType {
@@ -24,6 +37,12 @@ func toServiceType(service string) ServiceType {
 		return OneDriveService
 	case SharePointService.String():
 		return SharePointService
+	case ExchangeMetadataService.String():
+		return ExchangeMetadataService
+	case OneDriveMetadataService.String():
+		return OneDriveMetadataService
+	case SharePointMetadataService.String():
+		return SharePointMetadataService
 	default:
 		return UnknownService
 	}
@@ -31,6 +50,10 @@ func toServiceType(service string) ServiceType {
 
 var ErrorUnknownCategory = errors.New("unknown category string")
 
+// CategoryType denotes what category of data the path corresponds to. The order
+// of the enums below can be changed, but the string representation of each enum
+// must remain the same or migration code needs to be added to handle changes to
+// the string format.
 type CategoryType int
 
 //go:generate stringer -type=CategoryType -linecomment
@@ -63,7 +86,8 @@ func ToCategoryType(category string) CategoryType {
 	}
 }
 
-// serviceCategories is a mapping of all valid service/category pairs.
+// serviceCategories is a mapping of all valid service/category pairs for
+// non-metadata paths.
 var serviceCategories = map[ServiceType]map[CategoryType]struct{}{
 	ExchangeService: {
 		EmailCategory:    {},

src/pkg/path/resource_path_test.go

@@ -233,6 +233,101 @@ func (suite *DataLayerResourcePath) TestDir() {
 	}
 }
 
+func (suite *DataLayerResourcePath) TestToServiceCategoryMetadataPath() {
+	tenant := "a-tenant"
+	user := "a-user"
+	table := []struct {
+		name            string
+		service         path.ServiceType
+		category        path.CategoryType
+		postfix         []string
+		expectedService path.ServiceType
+		check           assert.ErrorAssertionFunc
+	}{
+		{
+			name:            "NoPostfixPasses",
+			service:         path.ExchangeService,
+			category:        path.EmailCategory,
+			expectedService: path.ExchangeMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:            "PostfixPasses",
+			service:         path.ExchangeService,
+			category:        path.EmailCategory,
+			postfix:         []string{"a", "b"},
+			expectedService: path.ExchangeMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:     "Fails",
+			service:  path.ExchangeService,
+			category: path.FilesCategory,
+			check:    assert.Error,
+		},
+		{
+			name:            "Passes",
+			service:         path.ExchangeService,
+			category:        path.ContactsCategory,
+			expectedService: path.ExchangeMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:            "Passes",
+			service:         path.ExchangeService,
+			category:        path.EventsCategory,
+			expectedService: path.ExchangeMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:            "Passes",
+			service:         path.OneDriveService,
+			category:        path.FilesCategory,
+			expectedService: path.OneDriveMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:            "Passes",
+			service:         path.SharePointService,
+			category:        path.LibrariesCategory,
+			expectedService: path.SharePointMetadataService,
+			check:           assert.NoError,
+		},
+		{
+			name:            "Passes",
+			service:         path.SharePointService,
+			category:        path.ListsCategory,
+			expectedService: path.SharePointMetadataService,
+			check:           assert.NoError,
+		},
+	}
+
+	for _, test := range table {
+		suite.T().Run(strings.Join([]string{
+			test.name,
+			test.service.String(),
+			test.category.String(),
+		}, "_"), func(t *testing.T) {
+			pb := path.Builder{}.Append(test.postfix...)
+			p, err := pb.ToServiceCategoryMetadataPath(
+				tenant,
+				user,
+				test.service,
+				test.category,
+				false,
+			)
+
+			test.check(t, err)
+
+			if err != nil {
+				return
+			}
+
+			assert.Equal(t, test.expectedService, p.Service())
+		})
+	}
+}
+
 func (suite *DataLayerResourcePath) TestToExchangePathForCategory() {
 	b := path.Builder{}.Append(rest...)
 	table := []struct {

src/pkg/path/servicetype_string.go

@@ -12,11 +12,14 @@ func _() {
 	_ = x[ExchangeService-1]
 	_ = x[OneDriveService-2]
 	_ = x[SharePointService-3]
+	_ = x[ExchangeMetadataService-4]
+	_ = x[OneDriveMetadataService-5]
+	_ = x[SharePointMetadataService-6]
 }
 
-const _ServiceType_name = "UnknownServiceexchangeonedrivesharepoint"
+const _ServiceType_name = "UnknownServiceexchangeonedrivesharepointexchangeMetadataonedriveMetadatasharepointMetadata"
 
-var _ServiceType_index = [...]uint8{0, 14, 22, 30, 40}
+var _ServiceType_index = [...]uint8{0, 14, 22, 30, 40, 56, 72, 90}
 
 func (i ServiceType) String() string {
 	if i < 0 || i >= ServiceType(len(_ServiceType_index)-1) {