Get a callback handler for the given mechanism name.
This method is called each time a mechanism is selected for the connection and the resulting
AuthorizingCallbackHandler will be cached and used multiple times for this connection, AuthorizingCallbackHandler
should either be thread safe or the ServerAuthenticationProvider should provide a new instance each time called.
Parameters:
mechanismName - the SASL mechanism to get a callback handler for
Returns:
the callback handler or null if the mechanism is not supported
