[Feature] Introduce native support for variant data type

[SteNicholas]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Variant was born from Apache Spark, and the encoding rules have been incorporated into Parquet https://github.com/apache/parquet-format/blob/master/VariantEncoding.md. Paimon has already supported variant data type in apache/paimon#4729.

Solution

Introduce native support for variant data type.

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[lxy-9602]

The variant type strikes a better balance between flexibility and performance for JSON storage and querying. Supporting it meaningfully enhances expressiveness and efficiency for semi-structured data.