Add support for reading variant type in Delta Lake

[ebyhr]

Description

The relevant classes can be moved to toolkit module or engine when we use it from Iceberg connector.

We should consider adding a new type variant instead of reusing json type.
I reused json type in this PR because old clients (e.g. CLI) can't print human-readable values for new types and I expect JSON functions will be used on the column.

• Protocol RFC https://github.com/delta-io/delta/blob/master/protocol_rfcs/variant-type.md
• Design doc https://docs.google.com/document/d/19QWDzSMFiWnpp3t7phsAWe1qV7tMv-uqtmJDxtMgK4A/edit?usp=sharing
• Variant type spec https://github.com/apache/spark/blob/master/common/variant/README.md
• Spark implementation https://github.com/apache/spark/tree/master/common/variant/src/main/java/org/apache/spark/types/variant
• PR [WIP][KERNEL][VARIANT] Add read in delta kernel delta-io/delta#3020

Fixes #22309

Release notes

# Delta Lake
* Add support for reading `variant` type. ({issue}`22309`)

[benbauer-draeger]

Hey, are there any plans to finish this feature as it would be highly useful?

[wendigo]

Didn't review copied/reformatted classes