Add Praefect service

This is the first Praefect specific RPC, which gets detail about all repositories and their replicas. We will use this to get status on which replicas are out of date, and for a rake task that gitlab admins can run to see the status of their repositories. (gitlab#35758 (closed))

note: this MR does not include the implementation yet. Waiting for !1627 (merged) to be merged