3454234: Restrict normalizers to custom_field data types.

Closes #3454234

src/Normalizer/EntityReferenceNormalizer.php

@@ -14,6 +14,8 @@ use Drupal\serialization\Normalizer\FieldItemNormalizer;
  */
 class EntityReferenceNormalizer extends FieldItemNormalizer {
 
+  protected $supportedInterfaceOrClass = 'Drupal\custom_field\Plugin\DataType\CustomFieldEntityReference';
+
   /**
    * The entity repository.
    *