API: Add unscoped `/merge_requests` endpoint

Description

Similar to the /issues endpoint, provide a /merge_requests endpoint that is not scoped to a project.

It would makes sense to do this, because the dashboard also allows accessing unscoped merge requests.

Proposal

• Add /merge_requests endpoint that returns all merge requests created by the authenticated user.
• Possible filters would be (similar to the Dashboard):
  • assignee_id
  • author_id
  • milestones
  • labels
• Ensure the docs are clear about "List merge requests" and "List project merge requests".

Links / references

• List issues
• List project issues
• List merge requests

Related issues

#29430 (closed)

Proposed documentation

List merge requests

Get all merge requests created by the authenticated user.

GET /merge_requests
GET /merge_requests?state=opened
GET /merge_requests?state=closed
GET /merge_requests?labels=foo
GET /merge_requests?labels=foo,bar
GET /merge_requests?labels=foo,bar&state=opened
GET /merge_requests?milestone=1.0.0
GET /merge_requests?milestone=1.0.0&state=opened
GET /merge_requests?iids[]=42&iids[]=43

Attribute	Type	Required	Description
state	string	no	Return all merge requests or just those that are opened or closed
labels	string	no	Comma-separated list of label names, merge requests must have all labels to be returned
milestones	string	no	Comma-separated list of milestone titles
iids	Array[integer]	no	Return only the merge requests having the given iid
assignee_id	integer	no	Return only merge requests assigned to the user with this id
author_id	integer	no	Return only merge requests authored by the user with this id
order_by	string	no	Return requests ordered by created_at or updated_at fields. Default is created_at
sort	string	no	Return requests sorted in asc or desc order. Default is desc

curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/merge_requests

cc @oswaldo @DouweM