Initial Client/Server Communication

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

Assets/Scripts/MessagePack/Utilities.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.Buffers;
+
+namespace MessagePack
+{
+    /// <summary>
+    /// Internal utilities and extension methods for various external types.
+    /// </summary>
+    internal static class Utilities
+    {
+        /// <summary>
+        /// A value indicating whether we're running on mono.
+        /// </summary>
+#if UNITY_2018_3_OR_NEWER
+        internal const bool IsMono = true; // hard code since we haven't tested whether mono is detected on all unity platforms.
+#else
+        internal static readonly bool IsMono = Type.GetType("Mono.Runtime") is Type;
+#endif
+
+        internal delegate void GetWriterBytesAction<TArg>(ref MessagePackWriter writer, TArg argument);
+
+        internal static byte[] GetWriterBytes<TArg>(TArg arg, GetWriterBytesAction<TArg> action)
+        {
+            using (var sequenceRental = SequencePool.Shared.Rent())
+            {
+                var writer = new MessagePackWriter(sequenceRental.Value);
+                action(ref writer, arg);
+                writer.Flush();
+                return sequenceRental.Value.AsReadOnlySequence.ToArray();
+            }
+        }
+
+        internal static Memory<T> GetMemoryCheckResult<T>(this IBufferWriter<T> bufferWriter, int size = 0)
+        {
+            var memory = bufferWriter.GetMemory(size);
+            if (memory.IsEmpty)
+            {
+                throw new InvalidOperationException("The underlying IBufferWriter<byte>.GetMemory(int) method returned an empty memory block, which is not allowed. This is a bug in " + bufferWriter.GetType().FullName);
+            }
+
+            return memory;
+        }
+    }
+}