RedshiftDriver support UNLOAD (exportBucket) in readOnly mode

Hello!

We started to support UNLOAD (exportBucket) in Redshift driver, but it's still limited and there are no support for readOnly mode.

Why?

Because We need column types for CSV, without it it's not possible to create a table in an external storage. In readOnly mode we dont create table before extraction and use original SQL, but there is no good way to do it.

Solution:

1. SELECT query with LIMIT 0 to extract types.
2. CREATE TEMP table and describe it via INFORMATION_SCHEMA.

Thanks