Add group members API endpoints to request access and approve requests

What does this MR do?

Add group members API endpoints to request access and approve requests.

Are there points in the code the reviewer needs to double check?

I chose to factorize the group/project members API as well as the new group/project access requests API.

I initially also created new services to centralize the permission checks related to actions on members, but this was out of scope and this MR is already quite big so I opened a temp MR at !5566 (closed) based on this one.

Why was this MR needed?

To finish the "request access" feature.

What are the relevant issue numbers?

Closes #18583 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together