Get milestone by title via API

Problem to solve

There are a bunch of APIs that require a milestone_id to be specified to set the value, for example https://docs.gitlab.com/ee/api/merge_requests.html#create-mr.

milestone_id | The global ID of a milestone

The only way to get the id is to list all the milestones for a given project (or group), and then select the one with the correct title attribute. Even with search, we cannot be sure we'll get only the result we need.

It would be easier to directly select a milestone by title, in a similar way we can select users by username:

GET /users?username=:username

Proposal

Improve current API calls to allow selecting a specific milestone by title

GET /projects/:id/milestones?title=:title
GET /groups/:id/milestones?title=:title