Add /groups/:id/subgroups endpoint to API
What does this MR do?
/groups/:id/subgroups endpoint to the API to list subgroups of a given parent group.
Does this MR meet the acceptance criteria?
- Changelog entry added, if necessary
- Documentation created/updated
- API support added
- Tests added for this feature/bug
- Has been reviewed by Backend
- Conform by the merge request performance guides
- Conform by the style guides
- Squashed related commits together