Resolve "Create 'Add to merge train when checks pass' service"

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/155034



Merged-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Approved-by: Tomas Vik <tvik@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Surabhi Suman <ssuman@gitlab.com>
Approved-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Reviewed-by: Patrick Bajao <ebajao@gitlab.com>
Reviewed-by: Tomas Vik <tvik@gitlab.com>
Reviewed-by: Hordur Freyr Yngvason <hfyngvason@gitlab.com>
Reviewed-by: Surabhi Suman <ssuman@gitlab.com>
Co-authored-by: Marc Shaw <mshaw@gitlab.com>
Co-authored-by: Phil Hughes <me@iamphill.com>