Add PdfSignedMessage and its serializer

Added a new class, PdfSignedMessage, to handle received signed PDF messages. Also, added a serializer for this class, PdfSignedMessageSerializer, for use with messaging. Furthermore, comment documentation has been added to RequestPdfSignMessage and its serializer for better clarity. Updated unit tests are also included.

src/Bundle/ChillDocStoreBundle/Service/Signature/Driver/BaseSigner/RequestPdfSignMessage.php

@@ -13,6 +13,9 @@ namespace Chill\DocStoreBundle\Service\Signature\Driver\BaseSigner;
 
 use Chill\DocStoreBundle\Service\Signature\PDFSignatureZone;
 
+/**
+ * Message which is sent when we request a signature on a pdf.
+ */
 final readonly class RequestPdfSignMessage
 {
     public function __construct(