Use backend code quality diff for MR widget: expose diff on MR widget entity, push feature flag to frontend

What does this MR do?

We're in the process of moving the code quality diff processing from the frontend to the backend 🎉 #271077 (closed)

This MR pushes the existing :codequality_mr_diff feature flag to the frontend and exposes the new backend diff endpoint from !48830 (merged) so that it can be used by the MR widget.

Does this MR meet the acceptance criteria?

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