net: mqtt: Add support for MQTT 5.0 AUTH packet

Add support for a new AUTH message introduced in MQTT 5.0.
This is a new mechanism specified by MQTT 5.0, which allows clients and
brokers for enhanced authentication in between CONNECT and CONNACK
exchange. An additional MQTT event (MQTT_EVT_AUTH) was specified which
is triggered when the AUTH packet arrives from the broker.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>