Add /groups/:id/subgroups endpoint to API

EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15142

What does this MR do?

Add an /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
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/32652