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

Start by checking the linked Parquet and Arrow work to determine whether the required VariantType support is available, then read Spark's python/pyspark/sql/variant_utils.py as the referenced implementation. Done means PyIceberg has an agreed VariantType implementation path once the PyArrow dependency supports it.

Written by the indexing model from the issue text.