Merge Request API endpoint should allow filtering by merge user ID

Proposal

Several customers have recently requested the ability to easily see how many merge requests a particular user has merged in a particular period of time. While it's possible to collect this data from the Events API (/users/:user_id/events?target_type=merge_request&action=merged), having similar functionality in the Merge Request API would be more predictable, especially since we already have filters for other similar parameters (reviewer_id, approver_ids, author_id, etc.).

The proposal here would be to add another filter parameter to the /merge_requests endpoint, so that this works:

/merge_requests?merge_user_id=:user_id&scope=all