Initial Client/Server Communication

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

Assets/Scripts/MessagePack/Resolvers/AttributeFormatterResolver.cs

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+// 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.Linq; // require UNITY_2018_3_OR_NEWER
+using System.Reflection;
+using MessagePack.Formatters;
+
+namespace MessagePack.Resolvers
+{
+    /// <summary>
+    /// Get formatter from <see cref="MessagePackFormatterAttribute"/>.
+    /// </summary>
+    public sealed class AttributeFormatterResolver : IFormatterResolver
+    {
+        /// <summary>
+        /// The singleton instance that can be used.
+        /// </summary>
+        public static readonly AttributeFormatterResolver Instance = new AttributeFormatterResolver();
+
+        private AttributeFormatterResolver()
+        {
+        }
+
+        public IMessagePackFormatter<T> GetFormatter<T>()
+        {
+            return FormatterCache<T>.Formatter;
+        }
+
+        private static class FormatterCache<T>
+        {
+            public static readonly IMessagePackFormatter<T> Formatter;
+
+            static FormatterCache()
+            {
+#if UNITY_2018_3_OR_NEWER && !NETFX_CORE
+                var attr = (MessagePackFormatterAttribute)typeof(T).GetCustomAttributes(typeof(MessagePackFormatterAttribute), true).FirstOrDefault();
+#else
+                MessagePackFormatterAttribute attr = typeof(T).GetTypeInfo().GetCustomAttribute<MessagePackFormatterAttribute>();
+#endif
+                if (attr == null)
+                {
+                    return;
+                }
+
+                if (attr.Arguments == null)
+                {
+                    Formatter = (IMessagePackFormatter<T>)Activator.CreateInstance(attr.FormatterType);
+                }
+                else
+                {
+                    Formatter = (IMessagePackFormatter<T>)Activator.CreateInstance(attr.FormatterType, attr.Arguments);
+                }
+            }
+        }
+    }
+}