RepositoryService: implement SetFullPath

This method is used to record the Project full path (part of URL, nothing to do with the file system) in the repository, for debugging purposes. Thanks to this, people and monitoring agents looking at Git repositories only can know whom to alert, where to look for visible damage etc.

In previous versions, the Rails app was just using the generic Git config support methods to store this information. In GitLab 14.2, there is a feature flag to use this dedicated gRPC method instead. The information is still stored in Git config, but we could use another mechanism for Mercurial.

If we implement this one and have the Rails app always use it (regardless of the feature flag) for Mercurial native projects, we will spare us the full implementation of the deprecated Git config support for fully native Mercurial without Git.