Entity bundles as plugins architecture

gistfile1.txt

- introduce a EntityBundle plugin type
- create a derivative class that provides the list of bundles for all entity types that
  a) have no default bundles
  b) have bundles defined by a config entity type
  
- the advantage of using the plugin system is that it automatically gives us a true entity sub-type concept
i.e.

/**
 * @Plugin(
 *   id = "node:page",
 *   label = @Translation("Page")
 * )
 */
class NodePage extends Node {
  ...

  some useful methods specific to the 'page' bundle.

  ...
}

 
- these subtypes can happily co-exist with bundles defined by a config entity type
i.e. the Node entity can have bundles managed by the NodeType config entity type but also the default NodePage bundle defined in code, as a plugin
 
 
- another goal is to use the EntityBundleManager for instantiating Entity objects
i.e. 

function entity_create($entity_type, array $values) {
  // Get the bundle from $values, or use a default one if it's not provided (let's say article for this example).
  $bundle = 'article';
  drupal_container()->get('plugin.manager.entity_bundle')->createInstance($entity_type . ':' . $bundle, array('values' => $values));
}


- the constructor of the Entity object calls the create() method from the appropiate storage controller. yes, this would switch the current order of the create operation.