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/Proxying/ConditionalProxyMiddleware.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.Http;
+using Microsoft.AspNetCore.Hosting;
+using System;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.SpaServices.Extensions.Proxy
+{
+    // This duplicates and updates the proxying logic in SpaServices so that we can update
+    // the project templates without waiting for 2.1 to ship. When 2.1 is ready to ship,
+    // merge the additional proxying features (e.g., proxying websocket connections) back
+    // into the SpaServices proxying code. It's all internal.
+    internal class ConditionalProxyMiddleware
+    {
+        private readonly RequestDelegate _next;
+        private readonly Task<Uri> _baseUriTask;
+        private readonly string _pathPrefix;
+        private readonly bool _pathPrefixIsRoot;
+        private readonly HttpClient _httpClient;
+        private readonly CancellationToken _applicationStoppingToken;
+
+        public ConditionalProxyMiddleware(
+            RequestDelegate next,
+            string pathPrefix,
+            TimeSpan requestTimeout,
+            Task<Uri> baseUriTask,
+            IApplicationLifetime applicationLifetime)
+        {
+            if (!pathPrefix.StartsWith("/"))
+            {
+                pathPrefix = "/" + pathPrefix;
+            }
+
+            _next = next;
+            _pathPrefix = pathPrefix;
+            _pathPrefixIsRoot = string.Equals(_pathPrefix, "/", StringComparison.Ordinal);
+            _baseUriTask = baseUriTask;
+            _httpClient = SpaProxy.CreateHttpClientForProxy(requestTimeout);
+            _applicationStoppingToken = applicationLifetime.ApplicationStopping;
+        }
+
+        public async Task Invoke(HttpContext context)
+        {
+            if (context.Request.Path.StartsWithSegments(_pathPrefix) || _pathPrefixIsRoot)
+            {
+                var didProxyRequest = await SpaProxy.PerformProxyRequest(
+                    context, _httpClient, _baseUriTask, _applicationStoppingToken, proxy404s: false);
+                if (didProxyRequest)
+                {
+                    return;
+                }
+            }
+
+            // Not a request we can proxy
+            await _next.Invoke(context);
+        }
+    }
+}