[client-backup] implement API call to get BackupID

Summary:
Call without auth to get latest BackupID.

Depends on D8993

Test Plan: Call this with mock data and verify it's lastly uploaded backupID

Reviewers: michal, marcin, bartek

Reviewed By: michal, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8994