Manual mocking responses for the http_client service

Implemented two new functions in the HttpClientFactoryStub for mocking responses as stack and dynamically:

  /**
   * Adds a custom response to the stack.
   *
   * @param \Psr\Http\Message\ResponseInterface $response
   *   A response to add.
   */
  public function stubAddCustomResponseToStack(ResponseInterface $response);

  /**
   * Sets a custom handler for the HTTP client.
   *
   * @param callable|null $handler
   *   A custom handler or null to unset the handler.
   */
  public function stubSetCustomHandler(?callable $handler);

Together with this, I added the "stub" prefix to all stub functions, to indicate that they are available only in the stub of the service.