HGitaly version: moved to inner file and exposed as part of ServerInfo method

This is the very first implementation of the Server gRPC service, just to expose the HGitaly version.

Because we still must support pip < 46 (Debian 10 in CI), the simplest way to work in all cases is to read the version from a separate file.

A first use-case would be to attempt conditional skipping of RSpec tests in the Rails app.