[services] Backup - Use Blob client in c++ - Implement add attachment

Summary:
Depends on D5017

Adding c++ implementation for using the blob client in the backup reactor for adding attachments.

Test Plan: backup builds

Reviewers: jon, tomek, max

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5018