XAdES-X-L v1.4.1 signature facet. This signature facet implementation will
upgrade a given XAdES-BES/EPES signature to XAdES-X-L.
We don't inherit from XAdESSignatureFacet as we also want to be able to use
this facet out of the context of a signature creation. This signature facet
assumes that the signature is already XAdES-BES/EPES compliant.
This implementation has been tested against an implementation that
participated multiple ETSI XAdES plugtests.