Add milestones, labels, and assignment to an MR in `Resource::MergeRequest.fabricate_via_api!`
The following discussion from !14336 (merged) should be addressed:
Ideally, this should implement all the options available in
fabricate!, i.e., milestones, labels, and assignment. The only use of those options is in
create_merge_request_spec.rb, which uses
fabricate_via_browser_ui!, so this won't cause any problems right now, but it should be updated sooner rather than later so we aren't surprised by the inconsistency later.