Config-specific entity type functionality moved to a new ConfigEntityTypeInterface

When working with entity types obtained from the entity manager the configuration entity types now implement the Drupal\Core\Config\Entity\ConfigEntityTypeInterface.

In general this should not impact any runtime code unless you are determining which entity types are configuration entity types using EntityType::getConfigPrefix(). That method has been moved to the Drupal\Core\Config\Entity\ConfigEntityTypeInterface. To discover if an entity type object is a configuration entity type just use instanceof. For example:

use \Drupal\Core\Config\Entity\ConfigEntityTypeInterface;

if ($entity_type instanceof ConfigEntityTypeInterface) {
  // Do config entity type specific thing.
}