[services] Backup - Use Blob client in c++ - Implement pull backup

Summary:
Depends on D5019

Adding c++ implementation for using the blob client in the backup reactor for pulling backups.

Test Plan: backup builds

Reviewers: jon, tomek, max

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5020