Allow to alter index name

Hi, I found that the settings form is always displayed on the search API index page.
But I just want to config it when I use the elasticsearch connector plugin.

I add the below code to hide the form when I use another search API.

  $searchIndex = $form_state->getFormObject()->getEntity();
  if (!$searchIndex->getServerInstance() || $searchIndex->getServerInstance()->getBackendId() !== 'elasticsearch') {
    return;
  }

The above patch has some issues. Update one.

The above patch has some issues. Update one.

The above patch has some issues. Update one.

@chilice Remember when uploading a patch to provide the intediff so it is easy to see the changes to do a review.