This is a required part of the PAdES spec, but so far we only wrote it.
As a start just expose if the attribute exists or not.

Change-Id: Iae3815f764973a2fd29d72593236c2f484172101
Reviewed-on: https://gerrit.libreoffice.org/31436Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>