Refactor BulkImportWorker logic into service

What does this MR do and why?

Refactor BulkImportWorker logic into service

This MR aims to address the second part of the code review suggestion upon which #327326 (closed) is based.

It's essentially a lift-and-shift of logic from a worker into a service object.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Enable local -> local requests.

   UPDATE application_settings SET allow_local_requests_from_web_hooks_and_services = true;

2. Perform a direct transfer from your GDK to itself.
3. Verify that group was transferred correctly.

MR acceptance checklist

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