Introduce a lightweight diffs_metadata endpoint

What does this MR do?

This introduces a lightweight diffs_metadata endpoint which will be used mainly to populate the File Tree and the diffs panel.

#31288 (closed)

Screenshots

Consistent low timings for serializing 114 diff file statistics

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

Performance 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