[Blazor] Add a way to specify a page as the Notfound fragment

[javiercn]

Differences in handling NotFound: On SSR scenarios this is represented by an endpoint, on the interactive router with a RenderFragment. We should extend the router to take in a NotFoundPage parameter that can be used during SSR and Interactive rendering to display not found content.

[javiercn]

The way we would do this is as follows:

• Expose a NotFoundPage parameter that takes in a Type which is the type of the page/component to render.
  • We should check that the type is an IComponent instance and that has a Route attribute applied to it, the goal is that it is friendly with https://learn.microsoft.com/en-us/aspnet/core/fundamentals/error-handling?view=aspnetcore-9.0#usestatuscodepageswithreexecute and https://learn.microsoft.com/en-us/aspnet/core/fundamentals/error-handling?view=aspnetcore-9.0#usestatuscodepageswithredirects
• If a user specifies both a NotFoundPage and a NotFound render fragment, the NotFoundPage wins.