Provisioning: delete secrets on repository deletion (#108113)

- Add hooks to git, github and github webhooks to remove the. 
- Implement deletion in secrets package.
- Add `Mutator` interface and hooks so that we can register any mutator. 
- Add unit test coverage to those mutators. 
- Move provider specific mutation from the massive `register.go` to the respective packages (e.g. `git` , `github`, etc). 
- Add integration test for removal. 
- Change the decryption fallback to simply check for the repository prefix.

pkg/registry/apis/provisioning/repository/git/git.go

@@ -12,6 +12,7 @@ type GitRepository interface {
 	repository.Writer
 	repository.Reader
 	repository.StageableRepository
+	repository.Hooks
 	URL() string
 	Branch() string
 }