Reuse the existing SearchBar in Stack v5 headers

[juliusmarminge]

V5 Stack headers currently only accept header item/spacer children, so the existing SearchBar cannot attach its UISearchController to the v5 UINavigationItem.

I propose an iOS-only Stack.HeaderConfig option, ios.searchBar, accepting the existing <SearchBar /> element. This follows the platform-specific v5 header configuration and preserves SearchBar props, events and ref commands. The native coordinator would attach/detach the search controller, refresh navigation-item options when SearchBar props change, and deactivate removed/replaced controllers. tvOS would ignore this option, as in the legacy header.

This first change would not add toolbar items, automatic search activation, or new SearchBar props. Johan Kasperi's #4013 already proposes legacy stack toolbars, Marco de Jongh's #4131 covers native-tabs search toolbars, and Konrad Michalik's #4679 covers UISearchTab. I would leave those concerns with their authors.

React Navigation's new native-stack adapter is still in #13115. A companion change there can map its existing headerSearchBarOptions to ios.searchBar without introducing another downstream option. I plan a minimal complete scenario and native before/after captures, including focus commands, filter menu actions, replacement/removal, and push/pop.

Does this fit the v5 header direction?