Gets the validation of the signature's time-stamp token: the first signature time-stamp attribute of a CMS signature, or, for a
document timestamp (ETSI.RFC3161), the token the signature is. null when there is none.
public PdfTimestampValidation? Timestamp { get; }
ISO 32000-2 §12.8.3.3.1, §12.8.3.4.3 b), §12.8.5; RFC 3161 Appendix A.