storing json in a columnar format

when storing a json, it would be beneficial to store it in a columnar format so each indexed field is treated like a column in a regular table.
then Pinot can possibly be able to also use different kinds of indexes on the values and not just inverted index, as well as other benefits of storing in columnar format (compression, etc)

No files, tests, or entry points are named. Start by identifying Pinot's existing JSON storage and indexing paths, then clarify the columnar representation, supported indexes, compatibility requirements, and measurable criteria for considering the feature complete.

Written by the indexing model from the issue text.