Add options builder methods for remote certificate validation callback.

Build/MQTTnet.nuspec

@@ -16,6 +16,7 @@
 * [Server] Exposed CONNECT packet properties in Application Message and Subscription interceptor.
 * [Server] Fixed: Sending Large packets with AspnetCore based connection throws System.ArgumentException.
 * [Server] Fixed wrong usage of socket option _NoDelay_.
+* [Server] Added remote certificate validation callback (thanks to @rudacs).
 * [MQTTnet.Server] Added REST API for publishing basic messages.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2019</copyright>

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Net;
+using System.Net.Security;
 using System.Security.Authentication;
 
 namespace MQTTnet.Server
@@ -98,7 +99,15 @@ namespace MQTTnet.Server
             _options.TlsEndpointOptions.IsEnabled = false;
             return this;
         }
-        
+
+#if !WINDOWS_UWP
+        public MqttServerOptionsBuilder WithRemoteCertificateValidationCallback(RemoteCertificateValidationCallback value)
+        {
+            _options.TlsEndpointOptions.RemoteCertificateValidationCallback = value;
+            return this;
+        }
+#endif
+
         public MqttServerOptionsBuilder WithStorage(IMqttServerStorage value)
         {
             _options.Storage = value;

Source/MQTTnet/Server/MqttServerTlsTcpEndpointOptions.cs

@@ -16,8 +16,9 @@ namespace MQTTnet.Server
 
         public bool CheckCertificateRevocation { get; set; }
 
+#if !WINDOWS_UWP
         public RemoteCertificateValidationCallback RemoteCertificateValidationCallback { get; set; }
-
+#endif
         public SslProtocols SslProtocol { get; set; } = SslProtocols.Tls12;
     }
 }