[Parquet] Support Variant Encoding for Parquet

[sfc-gh-mbojanczyk]

Describe the enhancement requested

Variant encoding looks like it's becoming part of the Parquet spec, and it'd be good to have Variant support in the Go library.

It looks like implementation of Variants is already under way in the Java Parquet library, as well as in Iceberg, so the spec seems to be settled enough that there shouldn't be massive changes in the base encoding. Looks like shredding still hasn't been tackled yet by any library, so that may be more in flux.

Variants will be a pretty powerful addon here for landing stuff in Iceberg- they generally have a good performance upside vs. using JSON or something similar.

Component(s)

Parquet