add missing vector object selector for multi-value string columns, refactor some stuff

[clintropolis]

Description

Adds missing VectorObjectSelector for multi-value STRING typed columns. Several complex aggregators use VectorObjectSelector when they don't care for dealing with or can't do anything useful with dictionary lookup stuff directly, so this fills in the missing selector implementation to ensure if it is called that it doesn't explode.

While I was here, this selector creating code is heavily copied between StringDictionaryEncodedColumn, StringFrontCodedDictionaryEncodedColumn, and even NestedFieldLiteralDictionaryEncodedColumn, so I have refactored the base machinery into abstract types and then each column implementation can bring its own dictionary to fill out how to do lookup, which was the primary difference between them.

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This PR has:

• been self-reviewed.
• 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.