Use redshift_connector's get_columns call to get column metadata

[mikealfare]

resolves #555

Problem

dbt-redshift references pg_catalog tables in its metadata queries. It should rely on Redshift's SDK or information_schema tables.

Solution

• Retire references to pg_catalog behind a behavior flag
• Move to SDK calls when possible
• Fall back on SQL calls that avoid referencing pg_catalog

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[mikealfare]

Make sure this name aligns with the naming convention: https://github.com/dbt-labs/dbt-core/blob/main/docs/guides/behavior-change-flags.md#rules-for-introducing-a-new-flag