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;