Add top-level /merge_requests API endpoint
What does this MR do?
Makes it possible to query for merge requests outside the scope of a project.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Documentation created/updated
- API support added
- Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
Branch has no merge conflicts with
master(if it does - rebase it please)
- Squashed related commits together
What are the relevant issue numbers?
Closes #28422 (closed).
Related to !13004 (merged).