[client-backup] implement Rust function for computing BackupKey

Summary:
ENG-4512.

Docs.

Function to compute (KDF) for hashing a password with BackupID.

Depends on D8884

Test Plan: cargo test in native_rust_library

Reviewers: michal, marcin

Reviewed By: michal, marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8885