Support AUTH packet exchange while connecting MqttClient

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:

https://github.com/dotnet/MQTTnet/blob/e18a91a4b59bc312aa9acf9be575401d07b793e4/Source/MQTTnet/Client/MqttClient.cs#L466-L469

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.