Add annotation for Sqoop Core

[Zhou, Richard]

Hi all:

I am working on https://issues.apache.org/jira/browse/SQOOP-2277 .
               All classes/interfaces in Sqoop Core component will be added 
such annotation, if they may be used by 3rd part code.


@InterfaceAudience.Public
@InterfaceStability.Unstable

                AFAIK, most security classes are in this list. Let me know if 
you want to add other classes in. Thanks.

List:
                AuthenticationHandler
                AuthenticationProvider
                AuthorizationAccessController
                AuthorizationHandler
                AuthorizationValidator


Regards
Richard