Add redshift external tables

[danieldiamond]

Description

Existing data-diff redshift capabilities are unable to read external tables. These are found in the svv_external_columns table.

Original thought was to add a database flag -e --external-table but I don't believe flags can be specific to one database connection and not the other. A simpler approach is to simply query the external tables in redshift if none was found in the redshift one.

• Adding select_external_table_schema in redshift sqeleton
• Add condition to query redshift external tables if no results were found in the redshift database.

[erezsh]

@danieldiamond Made a small change so the code is more modular.

#345

Please check that it works for you, and I'll merge it in.

If you can write a test for it, that would be even better! (maybe here)

[danieldiamond]

@erezsh was running into issues with the query_table_schema function, updated here

[erezsh]

(don't worry about the CI error, it's a problem on our end)

[erezsh]

Thank you for contributing!