Add NotFoundPage to Router

[ilonatommy]

Description

• Adds NotFoundPage parameter that can be used to customize the view of page rendered on NotFoundEvent.
• Fixes the configuration without global router: then no subscription to _notFound was done, so we use redirection to a hardcoded not-found page.
• Fixes the SSR streaming configuration. In streaming scenario we typically want to set 404 after the response already started (e.g. after request to a db). Throwing an exception in this case does not look good. Instead, we ignore the 404 there and redirect to NotFound page.
• If SSR streaming did not start or is not enabled, we can reexecute the request and set the result url to a chosen path, using app.UseStatusCodePagesWithReExecute("/reexecution-path", createScopeForErrors: true);

Fixes #58815

[javiercn]

Looks great!

I think we will also want to update the project templates to use this feature.

In addition to it, I think there's another scenario we want to enable here which is to make sure that it works with UseStatusCodeWithReexecute

• UseStatusCodeWithReexecute allows the app to render a page after a 404 has been set somewhere. We want that to also be possible with a Component page

I did something similar for handling errors a couple releases back. This is the PR with the change for reference #50550

• I suspect we will need to do something similar here.

/cc: @danroth27

[javiercn]

What would happen if we just remove httpContext.Response.StatusCode == StatusCodes.Status404NotFound and use only _hasStatusCodePage?

[ilonatommy]

It works but before doing so, I would like to clear the situation about re-executing what was not intended to re-execute see test.

[javiercn]

Is this effectively disabling streaming renders in 404 responses? Is there a reason for us to do it?

[ilonatommy]

If we start writing the response, we are effectively blocking the middleware's re-execution, a similar case to
#60970 (comment)
but with context.Response.HasStarted.