nested column support for ORC

[clintropolis]

Description

This PR adds support for Druid nested columns and JSON functions to ORC input files, fixing up the flattening conversion machinery to handle nested outputs, and fixing a few other conversion related bugs.

Release note

ORC input files are now supported by Druid nested columns and associated JSON transform functions.

---

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[paul-rogers]

The code and tests look fine. Can't comment on the detailed semantics. I'm trusting that the extensive unit tests prove that the semantics are valid.