[PP-1] Integrate the new Navigation Icon API with NavigationLinkBlock

This issue is still valid. We continue relying on the magic class pattern approach here. In my mind, ideally, we would allow the user to configure a icon_pack_id and icon_id pair for the block plugin that would get used in rendering it. That's an awkward UX experience, given core doesn't have any icon widgets, but, it could be left to contrib to make it better, and/or if/when core adopts a widget for icons.