First HGitaly specific service, MercurialRepositoryService

This is a starting point for Mercurial specific methods that are considered to be global with respect to a given repository.

It should cover the needs of #60 (closed) and #62

We should exert some caution, because of gRPC/protobuf general practices never to remove a field in a message for potential backwards compatibility with micro-services that could be mostly intractable.

In our case, we are not yet at the point where compatibility of components is a real problem: in our current Heptapod distribution options, we still bundle the exact appropriate HGitaly version with its clients. In practice, before the end of %HGitaly3: fully in charge and the subsequent split-up deployments, backwards compatibility of protocol is more of a development concern.

I followed the general Gitaly style: each method gets its own pair of request/response messages, even if they look like they could be mutualized.