API break: Add getViewsDisplay method to the Search API Facet Source

In #2859139: Automatically disable caching of views facet sources (regression) we noticed that there's code duplication in the module, we do something like this a couple of times. And we'll probably do this more if we want to handle views-specific stuff.

Because of this, it's may be a good idea to add a getViewsDisplay method to the \Drupal\facets\FacetSource\SearchApiFacetSourceInterface class.
Because we use this class also for integration with the search_api_pages modules we have to find a way to do this that allows us to return FALSE or something similar from that method.

      $facet_source_display_id = $facet_source->getPluginDefinition()['display_id'];
      $search_api_display = \Drupal::service('plugin.manager.search_api.display')
        ->createInstance($facet_source_display_id);
      $search_api_display_definition = $search_api_display->getPluginDefinition();

      if (!empty($search_api_display_definition['view_id'])) {
        $view_id = $search_api_display_definition['view_id'];
        $view_display = $search_api_display_definition['view_display'];

        $view = Views::getView($view_id);
        $view->setDisplay($view_display);
     }

Todo:
- Discuss if we want this.
- Implement in the interface
- Implement the code
- Overwrite the current usages of this code to the new code
- Write an additional test.