Summary:
Part of [ENG-10112](https://linear.app/comm/issue/ENG-10112/generate-user-keys-backup-an-a-client-during-restoration) and [ENG-10113](https://linear.app/comm/issue/ENG-10113/send-the-user-keys-backup-to-identity-service-in-a-single-rpc)

During restore, we might need to upload SIWE backup message along with User Keys so storing it to use exactly the sami API as during standard User Keys upload.

Depends on D14259

Test Plan: Restore still works for SIWE users

Reviewers: bartek, tomek