[Blazor] Lazy loading improvements for Webassembly

* With prerendering, it can happen that we prerender a path that requires lazy loaded assemblies to render. The current APIs don't offer a way to make sure those assemblies are loaded before the page tries to render, which results in failures when the app goes interactive. We need:
* A `LazyAssemblyLoader` implementation for prerendering (if you try to do this on any current .NET version it will fail).
* A way to run the lazy loading logic before we try to render the page in webassembly.

Alternatively, can the runtime provide a way to handle lazy loading assemblies