chore: initialize standalone UPM package

Runtime/Metadata/ShrinkNetworkMessageContracts.cs

@@ -0,0 +1,16 @@
+namespace ShrinkNetwork
+{
+    public interface IShrinkNetworkMessage
+    {
+    }
+
+    public interface IShrinkNetworkRequest : IShrinkNetworkMessage
+    {
+    }
+
+    public interface IShrinkNetworkResponse : IShrinkNetworkMessage
+    {
+        int ErrorCode { get; set; }
+        string ErrorMessage { get; set; }
+    }
+}