Provide a more intelligent routing fallback for Blazor apps that only falls back if the route matches a routable component

[danroth27]

The Blazor fallback logic always falls back to the specified route even if the original route doesn't match any routable components. This can be problematic in some cases, like in ASP.NET to ASP.NET Core app migration scenarios where you want to allow any unhandled requests to be proxied elsewhere. The Cocoon project currently uses some custom logic to identify all of the routable components and then only falls back for the component routes. We'll need something similar to support migrating to Blazor in our own incremental migration support based on YARP. We could provide some helpers that make this logic easier.

[SteveSandersonMS]

Sounds fine.

Implementation note: this would need to be an option, not a new default fallback behavior, because in general developers aren't required to use the built-in routing system and they might have a custom one of their own. We wouldn't be able to detect which URLs are matched by a custom router.

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