Bluetooth: controller: Add support for connectable ext adv

This adds handling for received AUX_CONNECT_REQ PDU which enables slave
connection with extended advertising.

Once AUX_CONNECT_REQ is received, standard checks are performed to
determine if we have resource to create connection, however unlike for
CONNECT_IND an rx node is temporarily kept in LLL until AUX_CONNECT_RSP
is successfully sent. This is to prevent creating a connection in case
response PDU was not sent for whatever reason.

A separate scratch buffer is created for AUX_CONNECT_RSP since default
radio scratch buffer is already used by received AUX_CONNECT_REQ.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>