Support AUTH packet exchange while connecting MqttClient

[vishnureddy17]

Currently, it appears that if the client sends a CONNECT packet using MQTT 5 extended authentication (i.e., it sets the Authentication Method) and it receives an AUTH packet from the server instead of a CONNACK packet, it will throw an exception:

MQTTnet/Source/MQTTnet/Client/MqttClient.cs

Lines 466 to 469 in e18a91a

	case MqttAuthPacket _:
	{
	throw new NotSupportedException("Extended authentication handler is not yet supported");
	}

This means that the client cannot connect using extended authentication with AUTH packets being exchanged. It looks like this feature gap is not being tracked, so I am opening this issue to track it.