[backup-client] make backupID mandatory when restoring backup

Summary:
ENG-6145.

Alternative, was to pass username/wallet to restoreBackup - but each time using this we should already have latestBackupInfo as it's needed during restore, so this better matches out protocol.

Depends on D13839

Test Plan: Tested in D13849

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13840