feat(auto-merge): detect merge requests waiting for

[mariusheine]

I as a user want to add a comment telling lassie to wait for another merge request to be merged. If this merge request got merged lassie shall merge the main branch into the current branch. On either success or failure the author of the wait shall be informed to check the changes by himself.

I as a user shall also be possible to remove this waiting.

It shall also be possible to wait for multiple merge requests.

[anbraten]

https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_dependencies.html

#premium