[commtest] Move backup client to shared
Summary:
ENG-5555 and ENG-5554
Move the backup client to shared so that it can be used on native. There are a few changes that I will annotate in inline comments. The total amount of moved logic is pretty small but if it's hard to review, let me know and I will split this into more diffs.
Test Plan:
- cargo run in backup and blob
- yarn run-integration-tests backup
- yarn run-performance-tests blob
Reviewers: bartek, varun, kamil
Reviewed By: bartek, varun
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D10260