Update controller and serializer entity for actually importing groups

What does this MR do?

This MR fixes passing data between BulkImportServiceController and relevant service to match expected format on frontend Also it fixes incorrect passing of session url. Without this fixes actual import #249160 (closed) via POST call was impossible

Alongside, it adds additional field to serializer to allow displaying links to relevant source groups as expected in #249157 (closed)

Screenshots (strongly suggested)

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