Part of [ENG-6145](https://linear.app/comm/issue/ENG-9644/backup-service-backup-client-work-for-a-minimal-version-of-backup).
Tracked in [ENG-9645](https://linear.app/comm/issue/ENG-9645/refactor-upload-method-into-upload-user-keys-and-upload-user-data).
1. Extract code that will be needed in endpoint introduced later in the stack
2. Added comment explaining purpose of this endpoint
3. Fixed typo in previous comment
Depends on D13872