Variant support in Iceberg

[AdamGS]

This is a tracking issue for Variant support for Iceberg, primarily for the Java implementation.

Motivation

Iceberg is one of the rare systems that supports variant, and more specifically the ParquetVariant encoding. Getting this support will move Vortex closer to spec compliance and is part of that effort.

Implementation history

• Add generic end-to-tend tests for Vortex spiraldb/iceberg#24