Initial Client/Server Communication

#1
Followed the MagicOnion quickstart guide, borrowing code from the chat sample app to get something working

Assets/Scripts/MessagePack/ExtensionResult.cs

@@ -0,0 +1,34 @@
+// Copyright (c) All contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.Buffers;
+
+namespace MessagePack
+{
+#if MESSAGEPACK_INTERNAL
+    internal
+#else
+    public
+#endif
+    struct ExtensionResult
+    {
+        public ExtensionResult(sbyte typeCode, Memory<byte> data)
+        {
+            this.TypeCode = typeCode;
+            this.Data = new ReadOnlySequence<byte>(data);
+        }
+
+        public ExtensionResult(sbyte typeCode, ReadOnlySequence<byte> data)
+        {
+            this.TypeCode = typeCode;
+            this.Data = data;
+        }
+
+        public sbyte TypeCode { get; private set; }
+
+        public ReadOnlySequence<byte> Data { get; private set; }
+
+        public ExtensionHeader Header => new ExtensionHeader(this.TypeCode, (uint)this.Data.Length);
+    }
+}