[client-backup] add local settings to store

Summary:
Allow user (for now only staff) to enable in disable automatic backup.

Toggle will be introduced in next diff.

Depends on D9003

Test Plan: Flip value and check result

Reviewers: michal, marcin, bartek, inka

Reviewed By: michal, inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9010