[client-backup] implement functions to decrypt backup

Summary:
Decrypting backup with AES-256 in GCM mode.

Depends on D8999

Test Plan: Encrypt/decrypt different types of both UserData and UserData.

Reviewers: michal, marcin

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9000