Add referer header for Algolia API restriction

Hi @phuang07

Have you filed a Pull Request with algolia/algoliasearch-client-php? I'm questioning whether changing vendors just for this single feature is justified.

@ivrh, they don't want to add a custom header to the default client config and that makes sense - https://github.com/algolia/algoliasearch-client-php/pull/713#issuecommen....

They suggest modifying the config for the client and this is a good solution. We need to wait for 📌 Add a way to access the algoliaClient Needs review and then we can use the hook alter to add own headers in custom modules:

use Algolia\AlgoliaSearch\Config\SearchConfig;

/**
 * Implements hook_search_api_algolia_search_client_config_alter().
 */
function my_module_search_api_algolia_search_client_config_alter(SearchConfig $config) {
  $config->setDefaultHeaders([
    'Custom-Header' => 'any value',
  ]);
}