NIFI-8973 Implement KerberosUserService API with keytab, password, and ticket cache implementations

[bbende]

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Also includes - NIFI-8974 Integrate KerberosUserService with HDFS processors

Enables X functionality; fixes bug NIFI-YYYY.

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[gresockj]

Looks like a nice improvement to streamline the Kerberos support, @bbende ! I just had some minor comments.

[exceptionfactory]

Thanks for putting this together @bbende, it looks like a great improvement to managing Kerberos authentication handling. The overall approach looks good, I noted a handful of small suggestions and questions.

[bbende]

@exceptionfactory thanks for the review, I think all comments should be addressed now, let me know if there is anything else

[exceptionfactory]

Thanks for the updates @bbende! Verified using GetHDFS with all three types of KerberosUserService implementations as well using existing properties. +1 Merging.