Support uploading nested data via CSV

CSV upload in Algolia dashboard does not allow to upload nested hierarchical data. The documentation says there is no agreed format for nesting data in CSV.

However, would you consider this tool to allow selecting columns which contain JSON to be uploaded as-is. E.g. you have a CSV in tabular format and then specific columns to be parsed as raw nested structures and be sent to Algolia like that.

What do you think?

| name | brand | categories |
| ---- | ----- | ---------- |
| Foo | Acme | `{"lvl0": "Home", "lvl1": "Seating", "lvl2": "Chairs"}` |

Start by reviewing the CSV upload flow and the documentation about nesting formats. Determine how column selection and JSON parsing should fit the existing upload behavior, then confirm that selected JSON columns are preserved as nested structures in Algolia records. The issue does not name specific files or tests.

Written by the indexing model from the issue text.