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/controller/mutator.go

@@ -0,0 +1,9 @@
+package controller
+
+import (
+	"context"
+
+	"k8s.io/apimachinery/pkg/runtime"
+)
+
+type Mutator func(ctx context.Context, obj runtime.Object) error