API support for managing merge request dependencies
## Shipped!
https://docs.gitlab.com/ee/api/merge_requests.html#get-merge-request-dependencies
## 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.
## Further details
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.
## Proposal
Add an API for managing merge request dependencies.
## Links / references
epic