Resolve "Allow filtering by `approved_by_usernames` in Group Merge Requests API"

What does this MR do and why?

Allow approved_by_usernames to be used in merge request apis

To test:

1. First find a group with merge requests
2. Choose a merge request and check that the current user has no approved it
3. Send a request with your token and group id, it should return empty
4. curl --request GET -H "PRIVATE-TOKEN: XX" "http://localhost:3000/api/v4/groups/xx/merge_requests" -F "approved_by_usernames[]=root"
5. Approve merge request, and rerun the request, and now it should return the MR

Related to #362722 (closed)