[services] Backup - Implement get method in service blob client

Summary:
Depends on D3630

Just calling Get method in ServiceBlobClient

Test Plan: The same as in D3535

Reviewers: palys-swm, geekbrother

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3632