Initial Client/Server Communication

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

Assets/Scripts/MessagePack/ExtensionHeader.cs

@@ -0,0 +1,33 @@
+// 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;
+
+namespace MessagePack
+{
+#if MESSAGEPACK_INTERNAL
+    internal
+#else
+    public
+#endif
+    struct ExtensionHeader : IEquatable<ExtensionHeader>
+    {
+        public sbyte TypeCode { get; private set; }
+
+        public uint Length { get; private set; }
+
+        public ExtensionHeader(sbyte typeCode, uint length)
+        {
+            this.TypeCode = typeCode;
+            this.Length = length;
+        }
+
+        public ExtensionHeader(sbyte typeCode, int length)
+        {
+            this.TypeCode = typeCode;
+            this.Length = (uint)length;
+        }
+
+        public bool Equals(ExtensionHeader other) => this.TypeCode == other.TypeCode && this.Length == other.Length;
+    }
+}