Create new top-level DefaultNodeInstance concept that will soon hold the "connection draining" logic

src/Microsoft.AspNetCore.NodeServices/Configuration/NodeHostingModel.cs

@@ -0,0 +1,8 @@
+namespace Microsoft.AspNetCore.NodeServices
+{
+    public enum NodeHostingModel
+    {
+        Http,
+        Socket,
+    }
+}