Consider changing Router lazy loading flow to be based on EventCallback<T>

[SteveSandersonMS]

This would be desirable because then developers can understand OnNavigateAsync as being equivalent to any other event handler, in that:

• It causes the host page to re-render both synchronously and asynchronously
• Any cancelled tasks will be discarded

If we don't do it, and keep it as Func<NavigationContext, Task>, then the host page won't re-render in response to navigation (only the Router and its descendants will). This is a bit of an obscure edge case, but would be surprising and would be hard to fix without a breaking change in the future.

Just for the sake of preserving where we got to, here's a quick sketch of how EventCallback could be used for this.

[SteveSandersonMS]

cc @captainsafia

[captainsafia]

It causes the host page to re-render both synchronously and asynchronously

This is roughly related to the problem outlined in #24211 in that we will need to to ensure that when a re-render is invoked we don't actually render the route component until the OnNavigateAsync task has completed. Specifically for the StateHasChanged call that is invoked when the task is started in our event handling logic (less so for when StateHasChanged is called once the async event callback is completed).

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