Support GitHub App auth for state store

[joebowbeer]

The docs say that Basic and SSH auth are the only means provided:

https://docs.kratix.io/main/reference/statestore/gitstatestore#auth

But GitHub App is arguably better because, among other reasons, it provides more granular control over actions. (Technically, GitHub App auth functions like Basic, but the token is generated by the GitHub App?)

GitHub App auth has been supported by ArgoCD (and Jenkins) for a while, and will be supported soon by Flux:

• https://github.com/argoproj/argo-cd/blob/master/docs/user-guide/private-repositories.md#github-app-credential
• [RFC-007] Implement GitHub app authentication for git repositories. fluxcd/source-controller#1647

[catmo-syntasso]

Thanks for raising the issue @joebowbeer - this is the first time we've had a request for GitHub App auth. We'll keep an eye on whether more teams request it!