Allow base field definitions to render

This is much apperciated, the patch didn't apply so I rewrote it a bit but otherwise it's as you suggested. Default is 'U'.

      $storage_format = match ($datetime_type) {
        DateTimeItem::DATETIME_TYPE_DATE => DateTimeItemInterface::DATE_STORAGE_FORMAT,
        DateTimeItem::DATETIME_TYPE_DATETIME => DateTimeItemInterface::DATETIME_STORAGE_FORMAT,
        default => 'U',
      };