Improve processors to support an optional key argument

Add extra $key = '' arg to CustomElementProcessorInterface::addtoElement() + CustomElementGenerator::process().

Closes #3443794

modules/custom_elements_thunder/src/Processor/ParagraphGalleryProcessor.php

@@ -29,7 +29,7 @@ class ParagraphGalleryProcessor implements CustomElementProcessorInterface {
   /**
    * {@inheritdoc}
    */
-  public function addtoElement($paragraph, CustomElement $element, $viewMode) {
+  public function addtoElement($paragraph, CustomElement $element, $viewMode, $key = '') {
     assert($paragraph instanceof ParagraphInterface);
     if ($this->fieldIsAccessible($paragraph, 'field_media', $element)) {
       /** @var \Drupal\media\Entity\Media $media_entity */