[client-backup] implement C++ function for computing BackupKey

Summary:
ENG-4512.

Note:
This is call to Rust, but it's related to cryptography, so I believe it should be executed on cryptoThread which is spawned via CommCoreModule - this is why I placed this here, not in CommRustModule. In the future, we might want to introduce new module to crypto-related things.

Depends on D8885

Test Plan: Call commCoreModule.computeBackupKey.

Reviewers: michal, marcin, varun

Reviewed By: marcin, varun

Subscribers: varun, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8886