Change Details

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

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 endpoints introduced later in the stack 2. Added comment explaining the purpose of this endpoint 3. Fixed typo in previous comment Depends on D13872

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 endpoints introduced later in the stack 2. Added comment explaining the purpose of this endpoint 3. Fixed typo in previous comment Depends on D13872