[backup-client] refactor SIWEBackupData type

Summary:
ENG-6145.

1. Move retrieving SIWEBackupData to a util funtion - it will be needed later in the stack
2. Move types to proper places on JS side
3. Add comment keep types in sync
4. Add validation on JS side
5. Remove siwe_backup_msg - it's not needed

Test Plan: Test backup restore for Wallet user

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13836