raft: Support adding read-only replicas to a partition

The epic &13562 doesn't aim to add routing and advanced Raft features. It focuses on integrating the Raft library into Gitaly as well as ensuring Raft replication is correct and robust. ie. replication of WAL logs between nodes. To reach that goal, Gitaly needs a simple way to add replicas to a partition or to all partitions of the storage.

To make it simple, we can implement a new sub-command to the existing gitaly binary. This command fires a gRPC request to the destination node. The server handles it from there. This issue scopes at adding a read-only replica. After a node joins, it needs to sync a huge amount of data before able to join the election. We'll add a way to promote a read-only replica to a fully functional replica in a later iteration.

The core replication is handled by #6401 (closed) and #6304.

Prerequisites:

• raft: Wire persistent routing table to Raft manager
• raft: Add membership management to Raft manager
• raft: Boot partition Raft group in replicas proactively
• raft: Add replication support (without autocompaction) to Raft manager
• raft: Promote learner node to voter node (#6803)