Add configuration for "as_sitesearch" attribute

Thank you for this module. I have been trying to get Google Custom Search Engine to work with limited success. This module seems like a cleaner implementation for Drupal 8 and is easier to build upon.

I would like to add support for the as_sitesearch attribute so that we can restrict results on some of our sites to a specific domain (our CSE includes many domains). Please see attached patch.