[services] Backup - Connect to Blob - split code to diferent files

Summary:
Depends on D4885

With this diff, I restructured previously added code so it'll be easier to understand what comes next.

Test Plan: backup still builds

Reviewers: varun, tomek, max, jon

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4888