Add support for blocking merge requests

Problem to solve

As a user, I want to be able to add/modify/delete blocking merge requests to new or existing merge requests.

Proposal

Please add support for the dependency APIs, i.e. https://docs.gitlab.com/api/merge_requests/#create-a-merge-request-dependency

Further details

We're using a process which requires merging to develop and main. When we do a release, we frequently need to merge to both from the same MR (to save time) and so we create them both and get them reviewed. Sometimes, people forget to manually put in the dependency and merge main before develop, deleting the branch.

It'd be nice to have a technical lock against shooting ourselves in the foot.