[services][backup] PullBackup 3/5 - response buffer utility

Summary:
First part of my attempt to separate transport and business logic, because it is terribly mixed in the C++ codebase.

This diff implements the ResponseBuffer utility struct which handles retrieving data chunks of arbitrary size. One of its functionalities is to reflect this C++ function.

Depends on D6241

Test Plan: This structure is used two diffls later and tested altogether with the whole endpoint. I'm wondering if it's worth adding unit tests here.

Reviewers: tomek, varun, jon, michal

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6242