Add support for `Started` and `Upcoming` milestone filtering to Issues APIs
Summary
While viewing list of Issues or MRs on either project level or group level, while selecting values for Milestone token, we support Started and Upcoming keywords to automatically apply relevant milestone as filter criteria. However, with our public REST APIs, we can only provide exact milestone title as value for milestone query params. Adding support for Started and Upcoming keywords will make the API on par with what we support on UI within GitLab.com.