chore: initialize standalone UPM package

Runtime/Transport/Abstractions/IShrinkNetworkAsyncTransport.cs

@@ -0,0 +1,9 @@
+using Cysharp.Threading.Tasks;
+
+namespace ShrinkNetwork
+{
+    public interface IShrinkNetworkAsyncTransport : IShrinkNetworkTransport
+    {
+        UniTask SendAsync(long sessionId, byte[] packetData);
+    }
+}