added benchmark for serializers

Frameworks/MQTTnet.NetStandard/Internal/MqttApplicationMessageExtensions.cs

@@ -2,7 +2,7 @@
 
 namespace MQTTnet.Internal
 {
-    internal static class MqttApplicationMessageExtensions
+    public static class MqttApplicationMessageExtensions
     {
         public static MqttApplicationMessage ToApplicationMessage(this MqttPublishPacket publishPacket)
         {

Frameworks/MQTTnet.NetStandard/Internal/TestMqttChannel.cs

@@ -0,0 +1,41 @@
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Channel;
+
+namespace MQTTnet.Core.Internal
+{
+    public class TestMqttChannel : IMqttChannel
+    {
+        private readonly MemoryStream _stream;
+
+        public TestMqttChannel(MemoryStream stream)
+        {
+            _stream = stream;
+        }
+
+        public void Dispose()
+        {
+        }
+
+        public Task ConnectAsync(CancellationToken cancellationToken)
+        {
+            return Task.FromResult(0);
+        }
+
+        public Task DisconnectAsync()
+        {
+            return Task.FromResult(0);
+        }
+
+        public Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _stream.ReadAsync(buffer, offset, count, cancellationToken);
+        }
+
+        public Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _stream.WriteAsync(buffer, offset, count, cancellationToken);
+        }
+    }
+}