core/dial: New channel variable FORWARDERNAME

Added a new channel variable FORWARDERNAME which indicates which
channel was responsible for a forwarding requests received on dial attempt.

Fixed a bug in the app_queue: FORWARD_CONTEXT is not used.

ASTERISK-26059 #close

Change-Id: I34e93e8c1b5e17776a77b319703c48c8ca48e7b2