Implementations of this interface are passed to a
CallbackHandler, allowing underlying security services
the ability to interact with a calling application to retrieve specific
authentication data such as usernames and passwords, or to display
certain information, such as error and warning messages.
Callback implementations do not retrieve or
display the information requested by underlying security services.
Callback implementations simply provide the means
to pass such requests to applications, and for applications,
if appropriate, to return requested information back to the
underlying security services.