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/Prerendering/ISpaPrerendererBuilder.cs

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+// 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.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.SpaServices.Prerendering
+{
+    /// <summary>
+    /// Represents the ability to build a Single Page Application (SPA) on demand
+    /// so that it can be prerendered. This is only intended to be used at development
+    /// time. In production, a SPA should already have been built during publishing.
+    /// </summary>
+    public interface ISpaPrerendererBuilder
+    {
+        /// <summary>
+        /// Builds the Single Page Application so that a JavaScript entrypoint file
+        /// exists on disk. Prerendering middleware can then execute that file in
+        /// a Node environment.
+        /// </summary>
+        /// <param name="spaBuilder">The <see cref="ISpaBuilder"/>.</param>
+        /// <returns>A <see cref="Task"/> representing completion of the build process.</returns>
+        Task Build(ISpaBuilder spaBuilder);
+    }
+}