Geo: Self Service Framework - Verify Merge request Diffs

Background

With &1817 (closed) complete, we need to add verification for the other blob types! This issue is for adding it to Merge request Diffs.

Definition of Done

• Merge request diff blobs are verified
• Verification for Merge request diff is enabled and tested on staging
• An issue is filed to coordinate the feature flag release

Procedure

Loosely From @mkozono on &5285 (comment 495424992)

1. Add primary-side verification fields to merge request diff details model table
2. Add secondary-side verification fields to merge request diff registry table
3. Return the status of Feature.enabled?(:geo_merge_request_diff_verification), from self.verification_feature_flag_enabled? for merge request diff replicator
4. Add to merge request diff registry class:

   include ::Gitlab::Geo::VerificationState
   include ::Geo::VerifiableRegistry

5. Test verification
6. Enable on staging, test with GitLab Environment Toolkit
7. Remove FF geo_merge_request_diff_verification