Add endpoint(s) for Git refs of a Repository and Runs for those refs

[mmurto]

As users are often interested in their repositorys' states for specific commits, having this data available in the API and UI would be great. Some ideas of what the endpoints could return:

• return all branches of a repository
• return all commits of a specified branch of a repository and runs for the commits that have runs
• return all runs for a commit

This would go towards allowing a use case where one branch (like main) could be set as a primary branch for compliance workflows, and the UI could show the latest run by commit timestamp rather than when the run was executed. If there were multiple runs for a commit, those should be sorted by execution timestamp.

For best use experience it would be good to have the commits and branches available before any runs for the repository have been executed, so these should probably be retrieved at Repository hierarchy item creation, and probably refreshed when loading the Repository again. This would also allow users to pick revisions to run scans for from a dropdown instead of text field without any information whether the revision exists.

What I have no idea of is what should be done for other VCSs than Git.