repository: Implement new `SetFullPath()` RPC

Since quite some time, we have tried to get rid of calls which modify
the on-disk gitconfig file. For one these include calls which add or
remove remotes, which are soon to be removed from Gitaly now that Rails
doesn't call them anymore. On the other hand, this also includes calls
which modify the gitconfig directly. There are currently two upstream
callers of those RPCs:

    - The first usecase is to set up JWT credentials for `FetchRemote()`
      calls. Those credentials are passed via HTTP headers, which is in
      fact already supported by `FetchRemote` without having to manually
      write them into the gitconfig. Those are thus easy to replace.

    - The second usecase is to set "gitlab.fullpath": this key simply
      records the repository's path in GitLab such that it's easy to see
      for admins what repository this is. This has been implemented to
      bridge the gap between the old storage layout and our new hashed
      storages.

This commit addresses the second usecase by introducing a new RPC
`SetFullPath()`. Instead of allowing callers to modify the gitconfig
arbitrarily, this will only allow callers to explicitly set
"gitlab.fullpath". With this in place, we can eventually remove
`AddConfig()` and `RemoveConfig()` given that they're not required
anymore.

Changelog: added