[Feature] Flux Extension: GitHub App auth

Since [Flux v2.5](https://fluxcd.io/flux/components/source/gitrepositories/#configure-github-app-secret), it is possible to bootstrap the Flux Operator using a GitHub app for authentication by creating a secret in the format detailed here:

Since GitHub Apps are independent of individuals, they are more maintainable than using e.g. PATs or personal SSH keys.

The latest Flux version used by the AKS Addon is (as of writing) already at v2.6.4, so the underlying capability should exist:

Feature request:

1. An implementation in the AKS flux controller that generates the GitHub App secret according to Flux expectations
2. Necessary updates to the [fluxConfiguration](https://learn.microsoft.com/en-us/azure/templates/microsoft.kubernetesconfiguration/fluxconfigurations?pivots=deployment-language-bicep) spec
3. Documentation updates