Support `directory` selection in xdp_portal_open_file()

[kasper93]

I would like to request and option to set this bool https://github.com/flatpak/xdg-desktop-portal/blob/71649cd9876072ed5c12a07928784b6c463a79eb/data/org.freedesktop.impl.portal.FileChooser.xml#L62-L64 when using xdp_portal_open_file()