[client-backup] implement API call to get UserData

Summary:
Call to get UserData associated with given BackupID.

Depends on D8995

Test Plan: Call this with mock data and verify result is correct.

Reviewers: michal, marcin, bartek

Reviewed By: michal, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8996