Update native backup code to handle siwe backup message
Summary: This diff modifies native rust code to attach backup message to upload request provided the file exists.
Test Plan: For now test that backup upload works for password users. Very next diff will enable testing for SIWE users with just one button click
Reviewers: kamil, bartek
Reviewed By: kamil
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D11715