RedshiftDataAPI: Serverless Support

[rmalecky]

The Redshift Data API recently added support for Redshift Serverless. WorkgroupName can now be specified instead of ClusterIdentifier.

Serverless also handles temporary credentials differently than provisioned capacity. The IAM principle is the database user so the db_user parameter isn't required. This will require an update to the logic that checks the auth credentials.

[kukushking]

Thanks @rmalecky it definitely makes sense to support that - we'll put that on the roadmap