Adds new method WithCustomDefaultMiddleware

To be able to fully customize the default middleware pipeline. Renames some interfaces and methods to be more consistent. Changes the startup builder interface types to be more type safe.

src/Umbraco.Web.UI.NetCore/Startup.cs

@@ -62,8 +62,8 @@ namespace Umbraco.Cms.Web.UI.NetCore
             app.UseUmbraco()
                 .WithMiddleware(u =>
                 {
-                    u.WithBackOffice();
-                    u.WithWebsite();
+                    u.UseBackOffice();
+                    u.UseWebsite();
                 })
                 .WithEndpoints(u =>
                 {