Included signatureZoneIndex for PdfSignedMessage in class definitions, handlers, and serializers to support signature zones. Updated test cases to reflect this new property, ensuring robust validation for handling and serialization.
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.
