Ingestion of column as array of array

### Description

Currently it is not possible to ingest a column as an array of array.
However, it is possible to create an ARRAY_AGG of ARRAY.

### Motivation

Rather than use ARRAY of json or string, it will be preferable to use directly ARRAY of ARRAY

The issue does not name any files, tests, or entry points. Start by tracing Druid's ingestion handling for array columns and the existing ARRAY_AGG of ARRAY behavior; the work is done when a column can be ingested directly as an array of arrays without using JSON or strings.

Written by the indexing model from the issue text.