override get_columns_in_relation with driver api call

[jiezhen-chen]

resolves dbt-labs/dbt-adapters#645

redshift__get_columns_relation currently is a hardcoded sql query that references the pg catalog. Changes in this PR leverage get_columns, a redshift_connector API method, to retrieve the same data.

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

[github-actions]

This PR has been marked as Stale because it has been open with no activity as of late. If you would like the PR to remain open, please comment on the PR or else it will be closed in 7 days.

[github-actions]

Although we are closing this PR as stale, it can still be reopened to continue development. Just add a comment to notify the maintainers.

[mikealfare]

This PR will be superceded by #899. @jiezhen-chen will be given contributor credit in that PR as the implementation is effectively the same as here.