[backup-service][commtest] implement integration tests for uploading User Keys and User Data separately

Summary:
Part of ENG-9673.

Tracked in ENG-9650.

I think I covered most of the cases, this is described in the code comments.

The endpoint to upload both User Keys and User Data at once is tested in backup_integration_test.

If anyone has any ideas about what test case can be added, I would be happy to do it.

Depends on D13878

Test Plan: Run tests

Reviewers: varun, bartek, tomek, angelika

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13879