[services] Backup - Add helper method for extra data

Summary:
Depends on D4554

Adding helper function PrepareDataChunkWithExtraData that performs necessary calculations. Context is in point 2 of this comment: https://phab.comm.dev/D4439#126984

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/D4571