Helper method to expose internal errors associated with a public error category (#3233)

<!-- PR description-->

Helper method to expose internal errors associated with a public error category, useful for testing `errs.Is` scenarios. The internal error needs to be used because `errs.Is` checks if the error chain contains a ref to the internal error.

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #COR-77

#### Test Plan

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

src/pkg/errs/err.go

@@ -29,6 +29,11 @@ var internalToExternal = map[errEnum][]error{
 	ResourceOwnerNotFound: {graph.ErrResourceOwnerNotFound},
 }
 
+// Internal returns the internal errors which match to the public error category.
+func Internal(enum errEnum) []error {
+	return internalToExternal[enum]
+}
+
 // Is checks if the provided error contains an internal error that matches
 // the public error category.
 func Is(err error, enum errEnum) bool {

src/pkg/errs/errs_test.go

@@ -19,6 +19,23 @@ func TestErrUnitSuite(t *testing.T) {
 	suite.Run(t, &ErrUnitSuite{Suite: tester.NewUnitSuite(t)})
 }
 
+func (suite *ErrUnitSuite) TestInternal() {
+	table := []struct {
+		get    errEnum
+		expect []error
+	}{
+		{RepoAlreadyExists, []error{repository.ErrorRepoAlreadyExists}},
+		{BackupNotFound, []error{repository.ErrorBackupNotFound}},
+		{ServiceNotEnabled, []error{graph.ErrServiceNotEnabled}},
+		{ResourceOwnerNotFound, []error{graph.ErrResourceOwnerNotFound}},
+	}
+	for _, test := range table {
+		suite.Run(string(test.get), func() {
+			assert.ElementsMatch(suite.T(), test.expect, Internal(test.get))
+		})
+	}
+}
+
 func (suite *ErrUnitSuite) TestIs() {
 	table := []struct {
 		is    errEnum