Add ability to get merge requests for a specific milestone via the API
Description
Right now, the API provides a way to GET the issues associated with a specific milestone in two different places:
/api/v3/milestones/:milestone_id/issues
/api/v3/projects/:project_id/issues?milestone=MILESTONE_TITLE
However, there does not appear to be any way to get the Merge Requests associated for a specific milestone.
Proposal
Add similar endpoints for listing merge requests. Because issues already supports this in two places, I think adding similar support for merge requests would be "least surprising".
/api/v3/milestones/:milestone_id/merge_requests
/api/v3/projects/:project_id/merge_requests?milestone=MILESTONE_TITLE