Add support for IServiceProviderFactory<TContainerBuilder>

- A new abstraction that allows the DI container author to describe how
to translate an IServiceCollection to a specific container and then to an IServiceProvider.
- Hosting will use this abstraction to swap out the default container

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

@@ -0,0 +1,24 @@
+using System;
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    /// <summary>
+    /// Provides an extension point for creating a container specific builder and an <see cref="IServiceProvider"/>.
+    /// </summary>
+    public interface IServiceProviderFactory<TContainerBuilder>
+    {
+        /// <summary>
+        /// Creates a container builder from an <see cref="IServiceCollection"/>.
+        /// </summary>
+        /// <param name="services">The collection of services</param>
+        /// <returns>A container builder that can be used to create an <see cref="IServiceProvider"/>.</returns>
+        TContainerBuilder CreateBuilder(IServiceCollection services);
+
+        /// <summary>
+        /// Creates an <see cref="IServiceProvider"/> from the container builder.
+        /// </summary>
+        /// <param name="containerBuilder">The container builder</param>
+        /// <returns>An <see cref="IServiceProvider"/></returns>
+        IServiceProvider CreateServiceProvider(TContainerBuilder containerBuilder);
+    }
+}

src/Microsoft.Extensions.DependencyInjection/DefaultServiceProviderFactory.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    public class DefaultServiceProviderFactory : IServiceProviderFactory<IServiceCollection>
+    {
+        public IServiceCollection CreateBuilder(IServiceCollection services)
+        {
+            return services;
+        }
+
+        public IServiceProvider CreateServiceProvider(IServiceCollection containerBuilder)
+        {
+            return containerBuilder.BuildServiceProvider();
+        }
+    }
+}