Transparently proxy SSH Git push from secondary to primary

Problem to solve

When running the fetch and push operations on a Geo secondary node using SSH, users currently have to first manually update the push URL everytime a new repository is cloned.

Proposal

We should make it possible for Geo users to use both fetch and push operations on a secondary by transparently proxying these git operations to the primary when using SSH.
This makes it much easier for users to use Geo because they only needed to grab the closest secondary when cloning and they're good to go.

What does success look like, and how can we measure that?

No manual changes of remote URLs are required when fetching/pushing after cloning a respository from a Geo secondary node.

Links / references

• Add SSH support following https://gitlab.com/gitlab-org/gitlab-ee/issues/3912.

/label ~"feature proposal"