Add IDesignTimeServiceProviderFactory

Provides access to the application's IServiceProvider to design-time services.

Resolves #524

Author: bricelam

src/Microsoft.Extensions.DependencyInjection.Abstractions/IDesignTimeServiceProviderFactory.cs

@@ -0,0 +1,22 @@
+// 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;
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    /// <summary>
+    /// A factory for creating the application's <see cref="IServiceProvider"/> at design time. Implement this interface
+    /// to enable design-time services that need access to the application's services. Design-time services will
+    /// automatically discover implementations of this interface.
+    /// </summary>
+    public interface IDesignTimeServiceProviderFactory
+    {
+        /// <summary>
+        /// Creates a new instance of the application's <see cref="IServiceProvider"/>.
+        /// </summary>
+        /// <param name="args">Arguments provided by the design-time service.</param>
+        /// <returns>The application's <see cref="IServiceProvider"/>.</returns>
+        IServiceProvider CreateServiceProvider(string[] args);
+    }
+}