[FEATURE] Send end event to client when an ASYNCAGI BREAK is executed on Asterisk

Not complete as this is currently a breaking change. Needs the addition of a check for a channel variable to switch on this behaviour.

This solves the use case where an application wishes to send a call back to the Asterisk dialplan (forgetting all about it in the mean time and ceasing to track it), and then potentially re-enter adhearsion via a further entry to asyncagi.