Fix(redshift): regression in varchar length workaround

[treysp]

NOTE: redshift integration tests won't pass until sqlglot is bumped > 25.24.4

#2791 introduced a regression in the varchar length workaround because it assumed the columns() method returned data types including their length/precision. That was true for MSSQL, but wasn't for Redshift.

This PR updates Redshift's columns() method to attach length/precision to data types.

Note: the _columns_query() method allowed postgres and redshift to use different columns queries inside postgres' columns() method. Redshift now needs to override the entire columns() method, so _columns_query() has been removed from postgres.