Annotate applicable mutator RPC's with target repos

This change introduces a new annotation for specifying which field contains the target repository for a mutator RPC that makes modifications to a specific repository. It also introduces the notion of an RPC "scope". The "scope" indicates how the RPC mutates the intended target (the entire Gitaly server, or the repository). This information is needed to properly schedule replications for Gitaly HA.

In addition to this, there is a linter to ensure contributors properly designate their RPC's.

Closes gitaly#1588 (closed)