Add support for Redshift data sharing tables

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:

```sql
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:

```sql
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 https://github.com/jakob/Postico/issues/382

Start with the Redshift data-sharing documentation and compare the table discovery behavior described for svv_tables, svv_all_tables, and information_schema.columns. Reproduce the issue on a consumer cluster containing shared tables, then trace the table-listing entry point; done means shared tables appear in Postico's tables list.

Written by the indexing model from the issue text.