[backup] Add endpoint to delete user data

Summary:
Added a DELETE /user_data/{user_id} Backup service endpoint that takes care of deleting all service user data.

Depends on D12970

Test Plan:
Created two backups for user, and some logs (used Commtest to generate data). Called the endpoint in Postman

• Verified that DDB data was deleted
• Verified that blobs were removed from Blob service
• Verified that only service-to-service token is accepted. HTTP 403 otherwise

Reviewers: kamil, marcin, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12971