Multiproject pipeline

And yet another repo for which we want to trigger schaap-stack on merge request events.

@veenboer : you might find this interesting.

BTW: I am not entirely happy with the merg_request_event triggers, since they typically lead to duplicate pipelines when pushing commits to a branch that has an open MR. It seems we have to deal with this until https://gitlab.com/gitlab-org/gitlab/-/issues/201845 is fixed.