Open Variant Type for semi-structured data

I've been starting to experiment with implementing the Open Variant Type [^1] in Rust / DataFusion. There is a specification and Java library for this, and Spark will release this type in 4.0. There are also plans to integrate this into table formats such as Delta Lake [^3] and Iceberg [^4]. This would be a high-performance data type for semi-structured data, designed for better OLAP performance than JSON or BSON (discussed in #7845). I've discussed a little bit in the Arrow repo about it's potential as an Arrow extension type [^2].

I'm working on creating an extension similar to [datafusion-functions-json](https://github.com/datafusion-contrib/datafusion-functions-json). If we could create a new repo `datafusion-functions-variant`, I'd be happy to develop that in the open.

[^1]: https://github.com/apache/spark/tree/master/common/variant
[^2]: https://github.com/apache/arrow/issues/42069
[^3]: https://www.databricks.com/blog/introducing-open-variant-data-type-delta-lake-and-apache-spark
[^4]: https://lists.apache.org/thread/xnyo1k66dxh0ffpg7j9f04xgos0kwc34