Support add/drop/modify columns

Why it matters

This feature enables schema evolution for Iceberg tables by allowing users to alter the table structure through adding, dropping, or modifying columns with simple data types. It addresses the need for flexible and dynamic schema management in Iceberg-backed ClickHouse tables, improving usability and integration with evolving data requirements.

How to use it

Users can perform schema changes on Iceberg tables by using the standard ALTER TABLE commands to add, drop, or modify columns with simple types. These operations are now supported and will be applied correctly to the underlying Iceberg metadata.