[Blazor] Pass assemblies registered in MapComponents to the Router component automatically.

There are two routers, one on the server (ASP.NET Core) and one on the client. We've merged the two implementations for the most part, but there are details that still cause confusion.
* The need to declare additional assemblies in two locations: We should make the endpoint in ASP.NET Core pass along the configured assemblies to the client project via persisted state or some other mechanism.