Support Redshift cross-database queries

Redshift has [introduced cross-database queries](https://aws.amazon.com/about-aws/whats-new/2020/10/announcing-cross-database-queries-for-amazon-redshift-preview/), and we should support those from Metabase.

**Links**
- product doc: _link to product doc_
- eng doc: _link to technical design doc_
- feature branch: `branch-name` _this should be the feature branch where this work will be done in. PRs will be delivered against this branch_
- Issue links: _list of issues this should close when this is delivered_

**Implementation Plan**

Version 2.x of the Redshift JDBC driver is required, which was already done under #14617. The only thing left is to add another JDBC URL parameter to enable this, and set up some testing schemas to verify it.

***ETA***: 03/31/2021

**How to Test This Feature Out**

Create an external schema as described [here](https://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_EXTERNAL_SCHEMA.html), then connect Metabase to the schema where that command was run, and tables from that external schema should show up and be queryable.

Start by locating the Redshift connection configuration and the code that builds its JDBC URL; the issue names no file or test. Reproduce the setup with an external schema created using CREATE_EXTERNAL_SCHEMA, then verify that its tables appear and can be queried through Metabase. The required JDBC parameter and testing schemas are the remaining implementation points.

Written by the indexing model from the issue text.