Allow SCM Provider to authenticate via Github App

[gzur]

AFAICT, the SCM Provider only supports authentication via Github Tokens.

This is problematic for organisations, since it leaves you with the following options:

1. Rely on a developer's Github Token - which means that if that developer leaves your organisation, your CD pipeline stops working.
2. Create a dedicated "robot" user and rely on a Github Token created for that user. While this sounds optimal, it actually introduces operational overhead that is, in my view, entirely unnecessary, especially since many people are already using Github Apps to authenticate ArgoCD.
3. Something I'm not thinking of?

It would be infinitely better to be able to rely on the Github App mechanism already provided by ArgoCD.

---

As an aside, it would be probably be best if there was a unified authentication mechanism across the generators (I'm looking your way git / pullRequest / scmProvider generators)