Add `VariantType`

### Feature Request / Improvement

The VariantType is a semi-structured type that's part of V3.

- Hopefully, the Parquet format will be released soon: https://lists.apache.org/thread/mr8krzdy8qslhhy5zbll53fd2hy6cpwg
- Once that's in, I'll be added to Arrow: https://github.com/apache/arrow/pull/45375
- I think for PyIceberg, we need to wait for PyArrow support.

There is a Python implementation in Spark that we could vendor into the project, but that will be pretty slow: https://github.com/apache/spark/blob/master/python/pyspark/sql/variant_utils.py

Thoughts?

Original proposal: https://github.com/apache/iceberg/issues/10392

Comienza revisando el trabajo enlazado de Parquet y Arrow para determinar si está disponible la compatibilidad necesaria con VariantType; después, lee Spark's python/pyspark/sql/variant_utils.py como la implementación de referencia. El trabajo estará terminado cuando PyIceberg tenga una ruta de implementación acordada para VariantType una vez que la dependencia PyArrow lo admita.

Escrito por el modelo de indexación a partir del texto del issue.