Add support for Praefect in cluster managing Gitaly nodes outside cluster

Summary

We should support running Praefect in the cluster, which manages Gitaly nodes running outside of the cluster.

This is slightly different from #2660 (closed) - that issue aims to support internal Praefect next to external Gitaly instances not managed by the internal Praefect.

Thanks @WarheadsSE for calling out this scenario.