Initial Client/Server Communication

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

Assets/Scripts/MessagePack/Internal/CodeGenHelpers.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;
+using System.ComponentModel;
+
+namespace MessagePack.Internal
+{
+    /// <summary>
+    /// Helpers for generated code.
+    /// </summary>
+    /// <remarks>
+    /// This code is used by dynamically generated code as well as AOT generated code,
+    /// and thus must be public for the "C# generated and compiled into saved assembly" scenario.
+    /// </remarks>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public static class CodeGenHelpers
+    {
+        public static byte[] GetEncodedStringBytes(string value)
+        {
+            var byteCount = StringEncoding.UTF8.GetByteCount(value);
+            if (byteCount <= MessagePackRange.MaxFixStringLength)
+            {
+                var bytes = new byte[byteCount + 1];
+                bytes[0] = (byte)(MessagePackCode.MinFixStr | byteCount);
+                StringEncoding.UTF8.GetBytes(value, 0, value.Length, bytes, 1);
+                return bytes;
+            }
+            else if (byteCount <= byte.MaxValue)
+            {
+                var bytes = new byte[byteCount + 2];
+                bytes[0] = MessagePackCode.Str8;
+                bytes[1] = unchecked((byte)byteCount);
+                StringEncoding.UTF8.GetBytes(value, 0, value.Length, bytes, 2);
+                return bytes;
+            }
+            else if (byteCount <= ushort.MaxValue)
+            {
+                var bytes = new byte[byteCount + 3];
+                bytes[0] = MessagePackCode.Str16;
+                bytes[1] = unchecked((byte)(byteCount >> 8));
+                bytes[2] = unchecked((byte)byteCount);
+                StringEncoding.UTF8.GetBytes(value, 0, value.Length, bytes, 3);
+                return bytes;
+            }
+            else
+            {
+                var bytes = new byte[byteCount + 5];
+                bytes[0] = MessagePackCode.Str32;
+                bytes[1] = unchecked((byte)(byteCount >> 24));
+                bytes[2] = unchecked((byte)(byteCount >> 16));
+                bytes[3] = unchecked((byte)(byteCount >> 8));
+                bytes[4] = unchecked((byte)byteCount);
+                StringEncoding.UTF8.GetBytes(value, 0, value.Length, bytes, 5);
+                return bytes;
+            }
+        }
+
+        public static ReadOnlySpan<byte> GetSpanFromSequence(in ReadOnlySequence<byte> sequence)
+        {
+            if (sequence.IsSingleSegment)
+            {
+                return sequence.First.Span;
+            }
+
+            return sequence.ToArray();
+        }
+
+        public static ReadOnlySpan<byte> ReadStringSpan(ref MessagePackReader reader)
+        {
+            if (!reader.TryReadStringSpan(out ReadOnlySpan<byte> result))
+            {
+                return GetSpanFromSequence(reader.ReadStringSequence());
+            }
+
+            return result;
+        }
+
+        public static byte[] GetArrayFromNullableSequence(in ReadOnlySequence<byte>? sequence) => sequence?.ToArray();
+
+        private static ReadOnlySpan<byte> GetSpanFromSequence(in ReadOnlySequence<byte>? sequence)
+        {
+            return sequence.HasValue ? GetSpanFromSequence(sequence.Value) : default;
+        }
+    }
+}