[client-backup] implement backup handler

Summary:
For now in UserData we store UserStore. Each time it changes we upload a new backup.

Depends on D9002

Test Plan:
Test both successful and failed uploads.

Reviewers: michal, marcin, bartek, inka

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9003