[services] Backup - pull backup - change logic to use att holders out of data section

Summary:
Depends on D4571

Make pull backup reactor works properly for "extra data" - more context in D4439. With this, we can work with the attachments as well.

Test Plan: full test plan in D4556, for now, the service builds

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4555