Implement JSON status for importers behind feature flag

What does this MR do?

This MR contains required backend changes for #26326 (closed)

It introduces new feature flag new_import_ui and implements unified JSON status format for all code importers.

Screenshots

N/A

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