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

Tests/MQTTnet.Benchmarks/SerializerBenchmark.cs

@@ -1,6 +1,7 @@
 using BenchmarkDotNet.Attributes;
 using MQTTnet.Packets;
 using System;
+using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -77,6 +78,8 @@ namespace MQTTnet.Benchmarks
 
             public bool IsSecureConnection { get; } = false;
 
+            public X509Certificate2 ClientCertificate { get; }
+
             public void Reset()
             {
                 _position = _buffer.Offset;

Tests/MQTTnet.Core.Tests/Mockups/TestMqttCommunicationAdapter.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Concurrent;
+using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -18,6 +19,8 @@ namespace MQTTnet.Tests.Mockups
 
         public bool IsSecureConnection { get; } = false;
 
+        public X509Certificate2 ClientCertificate { get; }
+
         public MqttPacketFormatterAdapter PacketFormatterAdapter { get; } = new MqttPacketFormatterAdapter(MqttProtocolVersion.V311);
 
         public long BytesSent { get; }

Tests/MQTTnet.Core.Tests/MqttTcpChannel_Tests.cs

@@ -39,7 +39,7 @@ namespace MQTTnet.Tests
 
                 await Task.Delay(100, ct.Token);
 
-                var tcpChannel = new MqttTcpChannel(new NetworkStream(clientSocket, true), "test");
+                var tcpChannel = new MqttTcpChannel(new NetworkStream(clientSocket, true), "test", null);
 
                 var buffer = new byte[1];
                 await tcpChannel.ReadAsync(buffer, 0, 1, ct.Token);