Resolve "Implement API endpoint for creating merge request dependency"

What does this MR do and why?

Allow users to create merge request dependencies through the API

To Test

• Create an MR
• Send a curl request (curl -X POST -H "PRIVATE-TOKEN: PRIVATE_TOKEN" "http://gdk.test:3000/api/v4/projects/:project_id/merge_requests/:merge_request_iid/blocks?blocking_merge_request_id=:blocking_merge_request_id" )
• Load the MR and check that the dependency is added

Related to #364161 (closed)