GC: const variables extended to include default container names (#886)

## Description
Moves constants to a file and adds more source definitions and backgrounds to constants.
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## Type of change

<!--- Please check the type of change your PR introduces: --->
- [x] 🐹 Trivial/Minor

## Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
*closes #823<issue>

## Test Plan
Const variables to be used in future PRs. No tests are available. 
<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/connector/exchange/exchange_vars.go

@@ -31,7 +31,18 @@ const (
 	// Default Folder Names
 	//------------------------
 	// Mail Definitions: https://docs.microsoft.com/en-us/graph/api/resources/mailfolder?view=graph-rest-1.0
+
+	// inbox and root
+	DefaultMailFolder    = "Inbox"
+	rootFolderAlias      = "msgfolderroot"
 	DefaultContactFolder = "Contacts"
+	DefaultCalendar      = "Calendar"
+
+	//---------------------
+	// Paging
+	//-----------------
+	// nextDataLink definition https://docs.microsoft.com/en-us/graph/paging
+	nextDataLink = "@odata.nextLink"
 )
 
 // descendable represents objects that implement msgraph-sdk-go/models.entityable
@@ -47,3 +58,9 @@ type displayable interface {
 	GetId() *string
 	GetDisplayName() *string
 }
+
+// container is an interface that implements both the descendable and displayble interface.
+type container interface {
+	descendable
+	displayable
+}

src/internal/connector/exchange/mail_folder_cache.go

@@ -12,19 +12,6 @@ import (
 	"github.com/alcionai/corso/src/internal/path"
 )
 
-const (
-	// rootFolderAlias is the per-user root container alias for the exchange email
-	// hierarchy.
-	rootFolderAlias = "msgfolderroot"
-	// nextDataLink is a random map key so we can iterate through delta results.
-	nextDataLink = "@odata.nextLink"
-)
-
-type container interface {
-	descendable
-	displayable
-}
-
 // cachedContainer is used for local unit tests but also makes it so that this
 // code can be broken into generic- and service-specific chunks later on to
 // reuse logic in IDToPath.