DuckDB support for vortex Variant #8115

This is a tracking issue for adding support for the Variant type in the Vortex extension for DuckDB

Motivation

Variant is useful, and DuckDB already has support for it in Parquet and in its own storage format.

Design

DuckDB has its own Variant encoding, we should be able to either store it or convert another encoding into it efficiently at runtime.
DuckDB already includes logic of converting a ParquetVariant array into their internal format, which we might want to re-use or re-implement.