Geo Replicables - Add REST API for the SSF

Proposal

We've implemented the Geo SSF actions first in GraphQL as it's the preferred way to be consumed by the Web UI.

In order to provide an interface that is friendlier to DevOps / System Administrators we should provide equivalent functionality through our REST API.

There is an existing implementation for /geo_replication/designs which we can base our implementation on: https://gitlab.com/gitlab-org/gitlab/blob/main/ee/lib/api/geo_replication.rb