Challenge a parked inbound REGISTER with a 401 digest auth request

Targets an inbound REGISTER surfaced via the sip.registration_attempt event (by its attempt_id). Sends a SIP 401 with a WWW-Authenticate digest challenge; the UA's credentialed re-REGISTER is verified against the supplied credential and, on success, bound and answered with 200 OK. Provide either password or ha1. The attempt must still be parked (it auto-accepts after the consult timeout).