ListRepos RPC for streaming all repos found on Gitaly storage

An RPC is needed that lists all repositories on a Gitaly node. This is needed for consistency checks, disaster recovery, and mass imports into Praefect.

The RPC is intended to only be used by Praefect, so it should not be exported outside the project.

The RPC should allow Praefect to request all repos on a specified storage, and all responses should be streamed back as the Gitaly filesystem is crawled.

service InternalGitaly {
  rpc StorageWalk (StorageWalkRequest) returns (stream StorageWalkResponse)
}