EntityOwnerTrait can be used as a default implementation of EntityOwnerInterface

EntityOwnerTrait can be added to entities to satisfy the requirements of EntityOwnerInterface. In order to make use of this trait, developers must:

• Add the 'owner' entity key to the entity annotation, specifying a field name of the owner.
• use Drupal\user\EntityOwnerTrait in their entity class.
• Add additional fields returned by static::ownerBaseFieldDefinitions to their base field list.

Examples of this trait in action can be seen on Comment, Node, File and Media.

The trait adds a default value callback `\Drupal\user\EntityOwnerTrait::getDefaultEntityOwner`, if you have been previously using a different callback, do not forget to add an update to fix exported configuration:

/**
 * Updates stale references to Drupal\YOURMODULE\Entity\ENTITY_CLASS::OLD_CALLBACK.
 */
function YOURMODULE_post_update_modify_base_field_author_override() {
  $uid_fields = \Drupal::entityTypeManager()
    ->getStorage('base_field_override')
    ->getQuery()
    ->condition('entity_type', 'ENTITY_TYPE')
    ->condition('field_name', 'FIELD_NAME')
    ->condition('default_value_callback', 'Drupal\YOURMODULE\Entity\ENTITY_CLASS::OLD_CALLBACK')
    ->execute();
  foreach (BaseFieldOverride::loadMultiple($uid_fields) as $base_field_override) {
    $base_field_override->setDefaultValueCallback('Drupal\YOURMODULE\Entity\ENTITY_CLASS::getDefaultEntityOwner')->save();
  }
}

To fix this manually, export your configuration, update the callbacks and then import again. See #3153455: UID base field override configs can still have old default value callbacks for more details.