||This Callback class provides a pluggable way of performing delegation.
||This CallbackHandler implementation obtains the previously received message from a
DelegationCallback object, and obtains a received token
(SAML/UsernameToken/BinarySecurityToken) from it to be used as the delegation token.
||This CallbackHandler implementation obtains a username via the jaxws property
"ws-security.username", as defined in SecurityConstants, and creates a wss UsernameToken
(with no password) to be used as the delegation token.