[backup-client][native_rust_library] implement creating User Keys backup

Summary:
ENG-9656.

1. Create prepare_user_keys_backup to extract shared code
2. Add new method to create User Keys backup.

Depends on D13930

Test Plan: Tested in D13937

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13931