Add a way to access the algoliaClient

MR !27 contains changes that allow altering the search client configuration. Now we can use hook alter. Here is an example:

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->setConnectTimeout(15);
  $config->setDefaultHeaders([
    'Custom-Header' => 'any value',
  ]);
}

too many changes, little hard to review

I will try and validate the code changes in my project