Parquet: Add Variant type support in Parquet module

[aihuaxu]

This is to enable writing Variant data to Parquet file and read from Parquet file in the format of struct of metadata and data.

This PR adds:

• A Parquet Variant implementation which currently uses Spark-Variant library as the dependency. We can reimplement inside the Iceberg-Parquet library to remove such dependency for now before that is moved to Parquet project.
• Add the Variant support in Parquet reader/writer.

Fix: apache#11178.