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

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -136,7 +136,7 @@ namespace MQTTnet.Server
                 await connection.StopAsync().ConfigureAwait(false);
             }
 
-            if (_sessions.TryRemove(clientId, out var session))
+            if (_sessions.TryRemove(clientId, out _))
             {
             }
 
@@ -296,7 +296,8 @@ namespace MQTTnet.Server
                 connectPacket.Password,
                 connectPacket.WillMessage,
                 clientAdapter.Endpoint,
-                clientAdapter.IsSecureConnection);
+                clientAdapter.IsSecureConnection,
+                clientAdapter.ClientCertificate);
 
             var connectionValidator = _options.ConnectionValidator;
 

Source/MQTTnet/Server/MqttConnectionValidatorContext.cs

@@ -1,4 +1,5 @@
-using System.Text;
+using System.Security.Cryptography.X509Certificates;
+using System.Text;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
@@ -11,7 +12,8 @@ namespace MQTTnet.Server
             byte[] password, 
             MqttApplicationMessage willMessage, 
             string endpoint, 
-            bool isSecureConnection)
+            bool isSecureConnection,
+            X509Certificate2 clientCertificate)
         {
             ClientId = clientId;
             Username = username;
@@ -19,6 +21,7 @@ namespace MQTTnet.Server
             WillMessage = willMessage;
             Endpoint = endpoint;
             IsSecureConnection = isSecureConnection;
+            ClientCertificate = clientCertificate;
         }
 
         public string ClientId { get; }
@@ -35,6 +38,8 @@ namespace MQTTnet.Server
 
         public bool IsSecureConnection { get; }
 
+        public X509Certificate2 ClientCertificate { get; }
+
         public MqttConnectReturnCode ReturnCode { get; set; } = MqttConnectReturnCode.ConnectionAccepted;
     }
 }