Apply some enhancements to AuthenticationConfiguration uniqueness.

• Add admonishing JavaDoc to useCallbackHandler to point out the importance of per-identity uniqueness of the callback handler

The following also may be possible and useful:

• Modify the AuthenticationConfiguration process to capture instances for Supplier-driven components at the time the configuration is used via the AuthenticationContextConfigurationClient
• Add a variation of useCallbackHandler which accepts a Supplier<CallbackHandler>, or a Function<T, CallbackHandler and a T, allowing constructor refs to be given