JSON structure parser

DRILL-6953, PR #1913 provides an EVF-version of the JSON reader. The original plan was to commit a first draft, with the feature disabled, then evolve from there. Review comments suggested we ensure that the first PR pass all unit tests with the feature enabled. Another comment suggested the JSON parser portion be reusable for other format or storage plugins. All good ideas.

The amount of change in the original PR was becoming too large. So, we'll go ahead and split the work into smaller portions.

This ticket is for the "JSON structure parser." This parser accepts the token stream from the Jackson parser and provides a framework to create a tree structure that describes the data. To keep the code simple, to simplify tests, and to make review easier, the structure parser does not directly integrate with the rest of Drill. Instead, it provides a number of listener interfaces which the next level implements. (The next level will be in another PR.)