ParticipantCallStatus and ReasonParticipantLeft attributes added to participant-leave Conference event

ParticipantCallStatus and ReasonParticipantLeft are now being returned as attributes on the participant-leave Conference status callback event. 

Previously, discerning the reason for a given participant's departure from a conference could be challenging; for example, if a participant was added using the /Participants API but failed to answer the incoming call, the application would need to tie together the call’s status for its callbacks with the 'participant-leave' callback.

The participant-leave event callback now contains all of the information required to discern the underlying call status for a participant, as well as the cause of a participant's departure from the conference.