Resolve "`Member.add_user`doesn't detect existing members that have requested access"

What does this MR do?

This merge request handle the case when an access requester is added to a group or project (via the members page or the API).

In Member.add_user, if an access requester already exists, we simply accept their request (and set the created_by, access_level and expires_at attributes if given).

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

I've taken the opportunity to cleanup the whole {Group,Project}Member.add_user* methods since it was quite a mess.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together
• A merge request against Enterprise Edition: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/744

What are the relevant issue numbers?

Closes #21983 (closed)