git: Introduce `ReferenceBackend` types

The repos in Gitaly use files reference backend, as we introduce the new reftable backend, it is important to identify which format the repository is using.

Introduce the ReferenceBackend type which denotes the reference backend used in the repository. This allows us to add backend specific information. Also add code for identifying the backend type for a provided repository.

Most of this is similar to how the git.ObjectHash mechanism works.

We then extend this to include regex formats used in UpdateRef specific to reference backend and allow more tests to run.

Closes #5835 (closed)