[backup][backup-client] Add auth to logs websocket

Summary:
Addresses 3 in ENG-8473.

• Added LogWSResponse::AuthSuccess
• Modified backup service handler to verify passed UserIdentity
• Modified backup client to wait for AuthSuccess message after sending auth request.

Depends on D12508

Test Plan:
Backup integration tests. Added some logging.
Also tested with COMM_SERVICES_SKIP_CSAT_VERIFICATION - auth message is still required but verification always succeeds.

Reviewers: kamil, marcin, varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12511