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

首先检查链接的 Parquet 和 Arrow 工作，以确定所需的 VariantType 支持是否可用，然后阅读 Spark's python/pyspark/sql/variant_utils.py，将其作为参考实现。完成的标准是：一旦 PyArrow dependency 支持 VariantType，PyIceberg 就有一条已达成共识的 VariantType 实现路径。

由索引模型根据 Issue 内容生成。