[backup-service] refactor uploading function in /backups endpoint
Summary:
Part of ENG-6145.
Tracked in ENG-9645.
- Extract code that will be needed in endpoints introduced later in the stack
- Added comment explaining the purpose of this endpoint
- Fixed typo in previous comment
Depends on D13872
Test Plan: run commtest
Reviewers: varun, bartek
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D13873