Introduce function to get path to a file to temporarily save backup message

Summary:
The backup message content will be temporarily persisted in a file after backup creation. Then the thread responsible for backup upload will read this file and attach
its content to upload request. Finally the file will be deleted.
This diff introduces utility functions to get teh path to save this file.

Test Plan: Call this function from any native/rust code.

Reviewers: kamil, bartek

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11714