support non-constant expressions for path arguments for json_value and json_query

[clintropolis]

Description

This PR adds support for using expressions to compute the json path argument for JSON_VALUE and JSON_QUERY functions. Note that when using a path expression that isn't a constant, Druid cannot take advantage of the specialized virtual column typically used, and so must fall back to a pure expression virtual column to compute the values from the 'raw' json blob, which will be much less performant than if the path is constant and known up front.

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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.

[gianm]

Logic looks good, but about code structure— the triply-nested if statements with long bodies make it hard to follow, so it'd be better to split this into multiple methods, or break out the inner classes, or something.