Add new Microsoft.AspNetCore.SpaServices.Extensions package to host new runtime functionality needed for updated templates until 2.1 ships

src/Microsoft.AspNetCore.SpaServices.Extensions/DefaultSpaBuilder.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Builder;
+using System;
+
+namespace Microsoft.AspNetCore.SpaServices
+{
+    internal class DefaultSpaBuilder : ISpaBuilder
+    {
+        public IApplicationBuilder ApplicationBuilder { get; }
+
+        public SpaOptions Options { get; }
+
+        public DefaultSpaBuilder(IApplicationBuilder applicationBuilder, SpaOptions options)
+        {
+            ApplicationBuilder = applicationBuilder 
+                ?? throw new ArgumentNullException(nameof(applicationBuilder));
+
+            Options = options
+                ?? throw new ArgumentNullException(nameof(options));
+        }
+    }
+}