Added `pipeline` parameter to MergeRequestsController

Related issue #241063 (closed)

What does this MR do?

• Extended Projects::MergeRequestsController#reports_response to support an additional parameter (pipeline, default: nil), and responds with 204 No Content with Polling Interval header when pipeline is provided and is pending?

• Added an additional argument head_pipeline for every call to reports_response method In EE::Projects::MergeRequestsController

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done