Support for custom signature tokens (eg SAML Token Profile)

Please add support for custom signature tokens, similar to what wss4j currently offers. This allows SoapUI to implement more complex outgoing security configurations, like descibed in the OASIS SAML Token Profile specifications (eg SAML Holder-of-Key authentication using STS tokens).

A pull request to implement this feature is already being submitted: https://github.com/SmartBear/soapui/pull/96 in February

Thank you.