Add external focus node constructor to Focus widget (#90843)

I've added a Focus.withExternalFocusNode constructor to the Focus widget (and the FocusScope widget) that makes it explicit that the widget's attributes won't affect the settings of the given focus node.

This is to help address #83023, which is a snag in the API that people run into occasionally.

This should help make it explicit when you want the widget attributes to take precedence, and when you don't.