hook_form_alter with #header support (patch)

Hi, It would be nice to to move the #header array to the module scope so we can use something like:

hook_form_search_api_index_fields_alter

eg:

/**
 * Add search_api_glossary settings to Search API Index configuration.
 *
 * Implements hook_form_FORM_ID_alter().
 */
function search_api_glossary_form_search_api_index_fields_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
.
.
.  
  $form['entity:node']['#header'][] = t('Glossary AZ');
.
.
.
}