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/PdfSignedMessage.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Chill\DocStoreBundle\Service\Signature\Driver\BaseSigner;
+
+/**
+ * Message which is received when a pdf is signed.
+ */
+final readonly class PdfSignedMessage
+{
+    public function __construct(
+        public readonly int $signatureId,
+        public readonly string $content
+    ) {}
+}