This is in analogue to authoredMergeRequests and assignedMergeRequests.

Support for this feature is complete at the model and  finder level, and
this change exposes that at the API level.

This supports filtering by review with:
  - User.reviewRequestedMergeRequests
  - User.assignedMergeRequests(reviewer_username:)
  - User.authoredMergeRequests(reviewer_username:)
  - Project.mergeRequests(reviewer_username:)