Add support for reading variant type in Delta Lake connector

[ebyhr]

• 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