Add task node metadata for arguments

Follow-up for #2730603: Implement hook_entity_property_info

In task/hosting_task.module we have the code below to work on. Goal is for e.g. rules to be able to create a task with arguments.
See also: #2603704: Add action to create a new task

/**
 * Implements hook_entity_property_info_alter().
 */
function hosting_task_entity_property_info_alter(&$info) {
  $task_properties = &$info['node']['bundles']['task']['properties'];

...

  // TODO: how should we expand these arguments?
  /*
  $task_properties['task_arguments'] = array(
    'label' => t('Task arguments'),
    'description' => t('The argument for the given task.'),
    // TODO list<text> is insufficient, list<struct> maybe the solution
    // see ie commerce/modules/price/commerce_price.module commerce_price_field_data_property_info
    'type' => 'list<struct>',
    'setter callback' => 'entity_property_verbatim_set',
  );
  */
}