Add support for Redshift data sharing tables

[aristocrates]

Info on data sharing: https://docs.aws.amazon.com/redshift/latest/dg/datashare-overview.html

Right now I have a "consumer" cluster that has read-only access to several tables from another cluster via data sharing, and I can see rows corresponding to these tables when I run this:

select * from svv_all_tables;

Note that svv_tables omits these rows, but svv_all_tables has them.

I can also see column information for all of those tables from this query:

select * from information_schema.columns;

But I cannot see the tables in the tables list in Postico - in fact on a cluster with just data sharing tables the tables list is blank, even though I can definitely see there is a table in svv_all_tables (and can query that table):

This is on Version 2.3 dev (9781)

Similar to #382