In-cluster Git interface

[stealthybox]

@hiddeco had an idea about interoperability with other GitOps projects that already have their own sync loops and internal support for git.

One of those projects is https://github.com/isaaguilar/terraform-operator.

It could potentially be useful for source-controller to serve an awkward, secondary git server so that Flux users can piggy-back off of source-controllers advanced source ref-policy, suspend, and filtering features when using these kinds of tools that support Git natively.

Third-party ecosystem integrations with source-controller and notification-controller are a community goal but this sort of gradual interoperability may or may not be worthwhile.

cc @stefanprodan