Geo support for external merge request diffs

Problem to solve

GitLab Geo does not replicate/verify external merge request diffs on secondary nodes. See https://docs.gitlab.com/ee/administration/geo/replication/#limitations-on-replicationverification.

https://docs.gitlab.com/ee/administration/merge_request_diffs.html

Related issues

#33758 (closed) https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7356