Added support for generic host based IWebHostBuilder (#1580)

- This adds an implementation of IWebHostBuilder as a facade over the IHostBuilder.
This removes the 2 container issue by executing the Startup.ConfigureServies and Startup.ConfigureContainer inline as part of building the IHostBuilder.
- The implementation is highly compatible implementation since it exposes the same IWebHostBuilder interface.
Existing extensions mostly work.
- There are some caveats with this approach.
    - Injecting services into Startup is not extremely constrained to the
    services availble on HostBuilderContext. This includes the IHostingEnvironment
    and the IConfiguration.
    - IStartup is broken when using this pattern because it isn't composable.
    - The IStartupConfigureServicesFilter and IStartupConfigureContainer The before
    and after filters added in 2.1 are also broken  because there's a single container (it could maybe be fixed by downcasting and doing something specific on the GenericHostBuilder instance).
    - Calling into IWebHostBuilder.Build will throw a NotSupportedException since
    this implementation is just a facade over the IHostBuilder.

Author: davidfowl

samples/GenericWebHost/Program.cs

@@ -1,10 +1,9 @@
-using System;
-using System.Net;
-using System.Threading.Tasks;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.Hosting;
+using Microsoft.AspNetCore.Hosting;
 
 namespace GenericWebHost
 {
@@ -19,22 +18,20 @@ public static async Task Main(string[] args)
                     config.AddJsonFile("appsettings.json", optional: true);
                     config.AddCommandLine(args);
                 })
-                .ConfigureServices((hostContext, services) =>
-                {
-                })
                 .UseFakeServer()
-                .ConfigureWebHost((hostContext, app) =>
+                .ConfigureWebHost(builder =>
                 {
-                    app.Run(async (context) =>
+                    builder.Configure(app =>
                     {
-                        await context.Response.WriteAsync("Hello World!");
+                        app.Run(async (context) =>
+                        {
+                            await context.Response.WriteAsync("Hello World!");
+                        });
                     });
                 })
                 .UseConsoleLifetime()
                 .Build();
 
-            var s = host.Services;
-
             await host.RunAsync();
         }
     }

samples/GenericWebHost/WebHostExtensions.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 System.Threading;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal class GenericWebHostApplicationLifetime : IApplicationLifetime
+    {
+        private readonly Microsoft.Extensions.Hosting.IApplicationLifetime _applicationLifetime;
+        public GenericWebHostApplicationLifetime(Microsoft.Extensions.Hosting.IApplicationLifetime applicationLifetime)
+        {
+            _applicationLifetime = applicationLifetime;
+        }
+
+        public CancellationToken ApplicationStarted => _applicationLifetime.ApplicationStarted;
+
+        public CancellationToken ApplicationStopping => _applicationLifetime.ApplicationStopping;
+
+        public CancellationToken ApplicationStopped => _applicationLifetime.ApplicationStopped;
+
+        public void StopApplication() => _applicationLifetime.StopApplication();
+    }
+}