[backup-service][backup-client] rename username -> user_identifier

Summary:
ENG-6145.

username is consfusing, as it could be the Wallet address too

Depends on D13807

Test Plan: run commtest

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13808