Add API to toggle attention request on Assignees/Reviewers in MRs
To support merge requests that require my attention we need a backend API to toggle this status for reviewers and assignees.
I think for this we should be following what was created for the re-request a review feature https://gitlab.com/gitlab-org/gitlab/-/blob/755a333dc99063a849926130cbdfdec075245543/app/models/merge_request_reviewer.rb#L4-11