code: public function boot()
      {
          // allow publishing off the config
          $this->publishes([
              __DIR__.'/config/kickbox.php' => config_path('kickbox.php'),
          ], 'kickbox');
  
          // load translation files
          $this->loadTranslationsFrom(__DIR__ . '/lang', 'kickbox');
  
          $this->app->booted(function($app) {
              // get validator and translator
              $validator = $app['validator'];
              $translator = $app['translator'];
  
              // setup custom kickbox validator
              $validator->extend('kickbox', function($attribute, $value, $parameters, $validator){
  
                  // fetch the api key from the config - which allows the config to be cached
                  $kickboxApiKey = config('kickbox.api_key');
  
                  // throw exception if the kickbox credentials are missing from the env
                  if( $kickboxApiKey == null ) {
                      // throw the custom exception defined below
                      throw new KickboxCredentialsNotFoundException('Please provide a KICKBOX_API_KEY in your .env file.');
                  }
  
                  // get kickbox key from users env file
                  $client = new Kickbox($kickboxApiKey);
                  return $client->kickbox()->verify($value)->body['result'] !== 'undeliverable';
              }, $translator->get('kickbox::validation.kickbox'));
  
          });
      }
docs: Bootstrap any application services.
  
  @return void