[RFC-007] Add support for GitHub App auth to `Provider` API

We should add support in the Provider for having a `.spec.secretRef` that points to a Kubernetes Secret containing GitHub App keys. When the provider `.spec.type` is set to `github` or `github-dispatch`, the controller should switch to GitHub App auth if the referred secret contains `githubApp` prefix keys.

xref: https://github.com/fluxcd/flux2/issues/4846 