[improve] enable XML signature validation

[chakl]

Re-enable XML signature validation.

This requires a version of crypto-lib that provides crypto:validate-signature-by-certfile().

[joewiz]

@chakl Is there a version of crypto-lib that provides the required function? If so, the best way to assert the dependency on the required version is to add the version number to https://github.com/eXist-db/existdb-saml/blob/master/expath-pkg.xml.tmpl#L4, either as a literal @version, or a range via @semver or semver-min.