code: public function sign($message)
      {
          $signatureParameters = $this->signatureParameters($message);
          $message = $message->withAddedHeader('Signature', $signatureParameters->string());
          $message = $message->withAddedHeader('Authorization', 'Signature '.$signatureParameters->string());
  
          return $message;
      }
docs: @param RequestInterface $message
  
  @return RequestInterface