Finalize variant and shredding spec

[aihuaxu]

Describe the enhancement requested

Per Parquet's requirements, we need at least two reference implementations to finalize the Variant logical type specification and here are the current status:

• Java already has the encoding and shredding implementations in place:

Variant Decoding
Variant Encoding
Variant Shredding Writer
Variant Shredding Reader

• Go also includes encoding and shredding support:

Variant Encoding/Decoding
Variant Shredding

Rust is currently working on the shredding implementation. In addition to these, we already have a full Variant implementation in Apache Iceberg, as well as in some closed-source engines. At this point, I think we have enough implementation coverage to move forward with finalizing the Variant spec.

[Tishj]

If it helps, DuckDB has also implemented the read portion of the VariantEncoding + VariantShredding specs, and can successfully read Variant parquet files produced by Apache Iceberg.

[aihuaxu]

If it helps, DuckDB has also implemented the read portion of the VariantEncoding + VariantShredding specs, and can successfully read Variant parquet files produced by Apache Iceberg.

Thanks for the information. That's very helpful.