Add BackupRepository RPC

Fixes #4940 (closed)

Defines an RPC to backup a single repository. This is a replacement for BackupRepos which I determined to be too complex to support through praefect. Instead of a stream of repositories, we now just process a single repo.

Adds new server-side backup configs. The GoCloudURL config was meant to mirror a similar config in gitlab-runner.

BackupRepos will be removed separately. See !5931 (merged)