Add BulkImport cancel API for instance admins

What does this MR do and why?

This MR adds an ability to cancel an ongoing Direct Transfer migration for admin users using the API.

Mentions #438281 (closed)

dt-cancel.mov

How to set up and validate locally

1. Start a new Direct Transfer and copy it's id
2. While Direct Transfer is in progress make a cancel request as admin at /api/v4/bulk_imports/.../cancel
3. Verify response has import marked as canceled
4. Make the same request as original user that does not have admin rights
5. Verify response is forbidden
6. Make the same request as anonymous
7. Verify response is unauthorized

Numbered steps to set up and validate the change are strongly suggested.