Challenge a ringing inbound SIP leg with a 401 digest auth request

Sends a SIP 401 Unauthorized carrying a WWW-Authenticate digest challenge on an unanswered inbound INVITE. The current leg is torn down (a leg.disconnected with reason="challenged" is published); the UAC's credentialed re-INVITE arrives as a new inbound call surfaced via leg.ringing with authenticated=true once VoiceBlender verifies the response against the supplied credential. Provide either password or ha1. An invalid retry is answered with 403 Forbidden and never surfaced.