AddSpaStaticFiles/UseSpaStaticFiles APIs to clean up the React template (or other cases where SPA files are outside wwwroot)

src/Microsoft.AspNetCore.SpaServices.Extensions/StaticFiles/ISpaStaticFileProvider.cs

@@ -0,0 +1,20 @@
+// 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.Extensions.FileProviders;
+
+namespace Microsoft.AspNetCore.SpaServices.StaticFiles
+{
+    /// <summary>
+    /// Represents a service that can provide static files to be served for a Single Page
+    /// Application (SPA).
+    /// </summary>
+    public interface ISpaStaticFileProvider
+    {
+        /// <summary>
+        /// Gets the file provider, if available, that supplies the static files for the SPA.
+        /// The value is <c>null</c> if no file provider is available.
+        /// </summary>
+        IFileProvider FileProvider { get; }
+    }
+}