Consolidate git implementations

[pjbgf]

The implementation of git operations are scattered around flux2, source-controller, image-automation-controller and this repository.
Resulting in code duplication in both core functionalities and their tests.

Consolidation steps:

• Provide a Flux interface to abstract away the underlying git library being used. (Review/Extend on https://github.com/fluxcd/source-controller/blob/main/pkg/git/git.go)
• Resolve the leak of implementation details
• Consolidate git implementations in a shareable module.
• Consolidate git test functions in a shareable module.
• Replace existing implementations
  • source-controller
  • image-automation-controller

[pjbgf]

This is merged into git-refactoring at both source-controller and image-automation-controller and should be closed once those branches merge into main.