Support Git reftable reference backend

[weihanglo]

Git v3 will have reftable backend by default, and libgit2 already merged a PR for reftable support on main branch libgit2/libgit2#7117. git2-rs should find a way to help test the integration, at least as pull request and see if it can be seamlessly integrated.

[weihanglo]

If people want to help, I guess we can create some PR like #1206 to have a taste of changes on main branch. That would minimize the time to release on our side when upstream releases any new versions.