[native][rust] Split download_backup to separate ct1 and ct2

Summary:
In secondary device auth, we only need to restore backup data (ct2) so I need to separate it in the download_backup()

Depends on D11332

Test Plan: Backup restoration still works as before - checked it via the backup dev menu.

Reviewers: michal, kamil

Reviewed By: michal, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11344