Geo: Add External MR diff replication

Merged Toon Claes requested to merge tc-external-mr-diff-ssf into master

What does this MR do?

This adds Geo replication for external Merge Request Diffs.

Closes #215137 (closed), #233829 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Edited by Michael Kozono