Entity reference selection handlers should extend a base class

From now on, a custom entity reference selection handler should extend the new introduced base class \Drupal\Core\Entity\EntityReferenceSelection\SelectionPluginBase (or a successor).

Changes

Plugins extending this new base class:

• Are able to provide sane defaults by implementing the ConfigurablePluginInterface::defaultConfiguration() method.
• Are able to set or get the plugin configuration by using the ::setConfiguration() and ::getConfiguration() methods (implemented from ConfigurablePluginInterface).
• Should pass handler settings, previously passed in plugin configuration under handler_settings key, in the root of configuration. The handler_settings level/namespace was deprecated. This is true for ::defaultConfiguration() but also for ::setConfiguration().

Example of how the structure of plugin configuration has changed (DefaultSelection):

Before:

[
  'target_type' => 'node',
  'handler' => 'default',
  'entity' => NULL,
  'handler_settings' => [
    'target_bundles' => [
      'article',
    ],
    'sort' => [
      'field' => 'title',
      'direction' => 'ASC',
    ],
    'auto_create' => FALSE,
    'auto_create_bundle' => NULL,
  ],
]

After:

[
  'target_type' => 'node',
  'handler' => 'default',
  'entity' => NULL,
  'target_bundles' => [
    'article',
  ],
  'sort' => [
    'field' => 'title',
    'direction' => 'ASC',
  ],
  'auto_create' => FALSE,
  'auto_create_bundle' => NULL,
]

Backward compatibility

Existing Entity reference selection handlers that are currently setting or getting the handler settings under handler_settings key, will continue to work but we recommend to move those settings under the root level of configuration, as soon as possible.

Important note

This change notice is about the selection handler plugin configuration (used to init the plugin) and not about the entity reference field configuration (stored as config entity).