Allow multiple input signatures as input for verification requests

Extend the cardinality of the SignatureObject in the VerifyRequest component to '0..n'. The current intention of the limitation to '0..1', keeping the core's processing model simple, is already contradicted by the possibility of having multiple signatures contained in XMLDSig and CMS documents.

The complexities of reporting the different verification results and details will not be adressed by the core. This will be adressed by a specific profile.