Add `VariantType`

[Fokko]

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: GH-45937: [C++][Parquet] Variant logical type definition arrow#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: apache/iceberg#10392

[jayceslesar]

apache/arrow#45937 is the issue in arrow I think we should be tracking

[Tishj]

Just checking, this page is wrong right?
https://iceberg.apache.org/status/?h=variant#data-types

It says Y for variant under PyIceberg

[Fokko]

@Tishj That should be N indeed, same for Iceberg-Go and Iceberg-Rust

[rambleraptor]

I know it'll be slow, but I'm happy to get a PR out using the Spark implementation and the can switch over for the Pyarrow release.