A "default" SecurityTokenServiceProvider implementation that defines the Issue and Validate
Operations of the STS and adds support for issuing and validating SAML Assertions, and
validating UsernameTokens and X.509 Tokens.
This CallbackHandler implementation is populated with SAML Beans by the SAMLTokenProvider, and is tasked
with setting them on a SAMLCallback object, which will be parsed (by WSS4J) into a SAML Assertion.