ari/pjsip: Make it possible to control transfers through ARI

ari/pjsip: Make it possible to control transfers through ARI

Introduce a ChannelTransfer event and the ability to notify progress to
ARI. Implement emitting this event from the PJSIP channel instead of
handling the transfer in Asterisk when configured.

Introduce a dialplan function to the PJSIP channel to switch between the
"core" and "ari-only" behavior.

UserNote: Call transfers on the PJSIP channel can now be controlled by
ARI. This can be enabled by using the PJSIP_TRANSFER_HANDLING(ari-only)
dialplan function.