Geo: Add External MR diff replication

Proposal

Add replication for External Merge Request diffs using the Geo Self Service Framework. This issue focuses on the backend implementation

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

• Add a new blob type by following https://docs.gitlab.com/ee/development/geo/framework.html#blob-replicator-strategy
• Make tests pass
• Smoke test by creating one of those blobs
• You should see that blob become replicated on your secondary

Documentation

• Raise follow up issues to improve the SSF documentation / functionality