Fixes: #434
This change introduces new APIs for registering Actors DI and with
the HTTP pipeline based on the ASP.NET Core Startup.cs model. This
replaces `UseActors()` in Program.cs and delivers better integration
between the ASP.NET Core framework and Actors.
The bug that motivated this change was that the actors HTTP handlers
were separate from the application's main routing table. This meant that
certain routing patterns were totally broken (such as Blazor Server)
when used in the same project as actors. There was no real way to fix
this without a change in how we register the framework to make it more
consistent with other features that build on ASP.NET Core.