Dereference Pushdown for Iceberg Connector

[phd3]

Similar to #1720 , we should push down the dereference expressions in the Iceberg connector.

Iceberg provides dotted name-based access to sub columns (e.g. string "a.b" to get the nested field id), so the implementation should be simpler compared to hive connector. I'm working on some refactoring in hive connector to extract out common code that can be reused for Iceberg connector dereference pushdown.