[services] Tests - Backup - Add attachments for backups

Summary:
Depends on D4637

Adding a piece of test for "add attachments for backup" functionality.

Test Plan:

cd services
yarn run-performance-tests backup 1

Reviewers: tomek, varun

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4632