feat(ios): add subtitle support for UIMenu header button submenus

[andreavrr]

Description

UIMenuElement.subtitle is a writable property available since iOS 15. This adds support for setting a subtitle on UIMenu (submenus) in header bar button items, mirroring the existing subtitle support already present for UIAction items (added in #3396).

The approach is straightforward: after creating the UIMenu via the existing initializer, we assign menu.subtitle if the subtitle key is present in the config dictionary.

UIMenu *menu = [UIMenu menuWithTitle:...];
NSString *subtitle = dict[@"subtitle"];
if (subtitle != nil && @available(iOS 15.0, *)) {
  menu.subtitle = subtitle;
}
return menu;

TypeScript: adds subtitle?: string to HeaderBarButtonItemSubmenu.

Use case

Showing the currently selected filter value below a submenu label — the same pattern used in Apple's own apps (e.g. Photos).

Changes

• ios/RNSBarButtonItem.mm — assign subtitle on UIMenu after creation (iOS 15+)
• src/types.tsx — add subtitle?: string | undefined to HeaderBarButtonItemSubmenu

Checklist

• Included code example that can be used to test this change
• Updated TS types