[client-backup] add JS function to compute BackupKey based on user credentials

Summary:
Call Argon2 protocol implemented in D8885.

Depends on D8997

Test Plan: Similar to tests implemented in D8885 but in JS.

Reviewers: michal, marcin, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8998