Redshift adapter behavior changes

restrict_direct_pg_catalog_access

Originally, the dbt-redshift adapter was built on top of the dbt-postgres adapter and used Postgres tables for metadata access. With this flag enabled, the adapter will use the Redshift API (through the Python client) if available, or query Redshift's information_schema tables instead of using pg_ tables.

While we don't expect any user-noticeable behavior changes due to this change, out of caution we are gating it behind a behavior-change flag and encouraging users to test it before it becomes the default for everyone.