API support for blocking merge requests
Problem to solve
I want to make my pipelines and automation scripts aware of these dependencies, but I can't because there is no API.
Follow-up to https://gitlab.com/gitlab-org/gitlab-ee/issues/9688
In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11600 + https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13506 we add blocking merge requests, rails controllers and a frontend using those controllers to control them. However, these entities should also be visible and controllable via both the REST and GraphQL APIs.
Add an API for managing merge request dependencies.