Add support for PostgreSQL IDENTITY columns (GENERATED ALWAYS/BY DEFAULT AS IDENTITY)

[babymum7]

Problem
ReadySet currently does not support PostgreSQL IDENTITY columns that use GENERATED ALWAYS AS IDENTITY or GENERATED BY DEFAULT AS IDENTITY syntax.

Current Behavior
When attempting to use ReadySet with tables containing IDENTITY columns, the snapshot/replication process fails or produces errors.

Example DDL that fails:
CREATE TABLE users (
id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
name TEXT NOT NULL
);

Example Log:
ERROR connection{addr=192.168.117.1:49196}: readyset_tracing: thread panicked panic=true thread="Adapter Runtime" payload="generated expr can't be None" location="readyset-sql/src/ast/column.rs:386:30" backtrace=disabled backtrace