/**
 * Implements hook_entity_field_storage_info().
 */
function {{ machine_name }}_entity_field_storage_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type) {
  if (\Drupal::entityManager()->getStorage($entity_type->id()) instanceof DynamicallyFieldableEntityStorageInterface) {
    // Query by filtering on the ID as this is more efficient than filtering
    // on the entity_type property directly.
    $ids = \Drupal::entityQuery('field_storage_config')
      ->condition('id', $entity_type->id() . '.', 'STARTS_WITH')
      ->execute();
    // Fetch all fields and key them by field name.
    $field_storages = FieldStorageConfig::loadMultiple($ids);
    $result = [];
    foreach ($field_storages as $field_storage) {
      $result[$field_storage->getName()] = $field_storage;
    }

    return $result;
  }
}