Allow to omit identity function

Why it matters

This feature simplifies the table definition process by removing the need to explicitly wrap partitioning columns with the identity function for Iceberg tables. It reduces verbosity and potential user errors when defining table partitions, enhancing usability and alignment with typical partition specification expectations.

How to use it

When creating or altering Iceberg tables, users can now specify partitioning columns directly without wrapping them in the identity function. Simply list the partition columns as usual without additional function calls.