Allow custom data attributes conversion

Having applied the patch from my merge request, here's an example of using an alter hook in a site that uses the CKEditor media resize module. I can check if there's an image style in use, if so load it, get the image width and then use that to set all the appropriate attributes for that module to do its thing. But either way I just remove the data-entity-embed-display attribute because it's never valid anyway in my case, so it just ends up using the default display mode.

/**
 * Implements hook_convert_entity_media_embed_data_attributes_alter().
 */
function lwb_base_cms_configuration_convert_entity_media_embed_data_attributes_alter(&$value) {
  if (stristr($value, 'data-entity-embed-display-settings')) {
    // Extract the JSON data from the string.
    preg_match('/data-entity-embed-display-settings="([^"]*)"/', $value, $matches);
    if (isset($matches[1])) {
      $json_data = $matches[1];
      // Decode an html entities, such as ".
      $json_data = html_entity_decode($json_data, ENT_QUOTES | ENT_HTML5, 'UTF-8');
      // Decode the JSON data.
      $data = json_decode($json_data, TRUE);
      if (is_array($data)) {
        $image_style = $data['image_style'] ?? NULL;
        if ($image_style) {
          // If there is an image style specified, load the image style
          // information.
          /**
           * @var \Drupal\image\ImageStyleInterface $image_style
           */
          $image_style = \Drupal::entityTypeManager()->getStorage('image_style')->load($image_style);
          if ($image_style) {
            // If using an image style that scales and/or crops the image, we
            // want to setup the media embed attributes to trigger the
            // ckeditor_media_resize module to resize it accordingly.
            $effects = $image_style->getEffects();
            $image_width = NULL;
            foreach ($effects as $effect) {
              $effect_config = $effect->getConfiguration();
              if ($effect_config['id'] === 'image_scale_and_crop' || $effect_config['id'] === 'image_scale') {
                // Get the final dimensions from the effect.
                $image_width = $effect_config['data']['width'] ?? NULL;
              }
            }
            if ($image_width) {
              $value .= ' class="image_resized"';
              $value .= ' style="width: ' . $image_width . 'px;"';
              $value .= ' data-media-width="' . $image_width . 'px"';
            }
          }
        }
      }
    }
  }
  if (stristr($value, 'data-entity-embed-display')) {
    // Either way remove the data-entity-embed-display attribute because we
    // do not want that to be used as a view mode for the entity.
    preg_replace('/ data-entity-embed-display="[^"]*"/', '', $value);
  }
}