Expose client certificate at MQTT connection validator. Make DualMode optional.

Source/MQTTnet/Adapter/IMqttChannelAdapter.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Formatter;
@@ -12,6 +13,8 @@ namespace MQTTnet.Adapter
 
         bool IsSecureConnection { get; }
 
+        X509Certificate2 ClientCertificate { get; }
+
         MqttPacketFormatterAdapter PacketFormatterAdapter { get; }
 
         long BytesSent { get; }