Expose *all* hidden field metadata via Asset API → Forms endpoints

As revealed by @Dennis Keefer​ on this thread, the Forms Asset API does not completely expose metadata for hidden fields, such as their default values and their runtime source (cookie, referrer URL param, or current URL param).

Only the field name is exposed (and the position, which is immaterial).

Unless there is some secret endpoint where these are shown, this must be added for the API to be useful for anything but partial updates of known forms, since this is one of the most commonly used forms features.