[services] Tests - Add pull backup

Summary:
Depends on D4227

Adding pull backup module. It returns an instance of the BackupData with obtained sizes so they can be compared with the original ones later.

Test Plan: Please see D4216

Reviewers: palys-swm, varun, geekbrother

Reviewed By: palys-swm, varun, geekbrother

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4228