Support for externally linked tables (#12741)

In Redshift (and other databases), foreign tables show up as "EXTERNAL
TABLE" - allow those to be synced in with Metabase.

Updates the Redshift JDBC driver due to changelog entry from Amazon that
was occurring and causing immense slowdowns in running the Redshift
driver tests:

    When the Java application uses com.amazon.redshift.api.PGTimestamp directly, an exception occurs.

Resolves #7833