Initial Client/Server Communication

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

Assets/Scripts/MessagePack/Formatters/NilFormatter.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.
+
+namespace MessagePack.Formatters
+{
+    public class NilFormatter : IMessagePackFormatter<Nil>
+    {
+        public static readonly IMessagePackFormatter<Nil> Instance = new NilFormatter();
+
+        private NilFormatter()
+        {
+        }
+
+        public void Serialize(ref MessagePackWriter writer, Nil value, MessagePackSerializerOptions options)
+        {
+            writer.WriteNil();
+        }
+
+        public Nil Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
+        {
+            return reader.ReadNil();
+        }
+    }
+
+    // NullableNil is same as Nil.
+    public class NullableNilFormatter : IMessagePackFormatter<Nil?>
+    {
+        public static readonly IMessagePackFormatter<Nil?> Instance = new NullableNilFormatter();
+
+        private NullableNilFormatter()
+        {
+        }
+
+        public void Serialize(ref MessagePackWriter writer, Nil? value, MessagePackSerializerOptions options)
+        {
+            writer.WriteNil();
+        }
+
+        public Nil? Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
+        {
+            return reader.ReadNil();
+        }
+    }
+}