Add `pipeline` parameter to MergeRequestsController

This is a dependency for fixing #238235 (closed)

backend Extend the Projects::MergeRequestsController#reports_response (https://gitlab.com/gitlab-org/gitlab/blob/master/app/controllers/projects/merge_requests_controller.rb#L430) to support additional parameter (pipeline, default: nil), and respond with the 204 No Content response with Polling Interval header (https://gitlab.com/gitlab-org/gitlab/blob/master/app/controllers/projects/merge_requests_controller.rb#L433) when pipeline is provided and is pending?,
backend In EE::Projects::MergeRequestsController (https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/controllers/ee/projects/merge_requests_controller.rb) in every place we are calling reports_response method add additional argument: pipeline,