The only implementation of css::xml::crypto::XXMLSignatureTemplate is
XMLSignatureTemplateImpl, so work with that directly instead of going
via UNO.

Change-Id: I85e2169a909b689620c2ce125a9653f9a6696f45
Reviewed-on: https://gerrit.libreoffice.org/40950Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>