Allow asynchronous rebase operations to be monitored

What does this MR do?

Currently, it's impossible to reliably track the status of an asynchronous rebase operation.

This MR introduces a MergeRequest#rebase_jid which operates similarly to MergeRequest#merge_jid and can be used for reliable tracking.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #54117 (closed)