Proposal: add Variant type to iceberg

[aihuaxu]

Feature Request / Improvement

As discussed in the mailing list, and described in this doc, I'd like to add Variant type to Iceberg.

We will adopt the Spark Variant spec and move the spec to Parquet - see discussion. There are some ongoing discussion on how Parquet will adopt the spec and implementation - seed the doc, but from Iceberg side, I would like to start the implementation assuming the spec is available and make the final changes when the spec is finalized.

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time

[manuzhang]

Since variant type is a major feature, this issue might be better labeled as proposal following https://iceberg.apache.org/contribute/#what-is-an-improvement-proposal