[services] Backup - pull attachment holders out of the data section

Summary:
Raised in https://phab.comm.dev/D4245#124304

This change will reduce the cycles when sending the data with attachments

Test Plan: None for now, services will build, as usual, this field is optional so not setting it doesn't break anything.

Reviewers: tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4438