Do not use ControllerBase as parent class for RestExampleClientController

The \Drupal\rest_example\Controller\RestExampleClientController class extends \Drupal\Core\Controller\ControllerBase, but the only method it uses from that class is config().

Instead of extending ControllerBase, RestExampleClientController should:

• Implement \Drupal\Core\DependencyInjection\ContainerInjectionInterface
• Use \Drupal\Core\Messenger\MessengerTrait
• Use \Drupal\Core\StringTranslation\StringTranslationTrait
• Define a property for the configuration factory service

RestExampleClientController actually accesses ControllerBase::$configFactory before it is initialized with the effect of accessing a method on NULL.
Changing the class as described here fixes that bug too.