Postgres_scan should be able to read a persistent secret

Similar to `ATTACH`, you should be able to use a persistent secret. My current and only option at the moment is to attach the whole database even though I am only interested in 1 table.
```sql
CREATE PERSISTENT SECRET my_persistent_secret (
TYPE postgres,
HOST 'localhost,
PORT 5432,
DATABASE my_db,
USER 'my_user',
PASSWORD 'my_password'
);
SELECT * FROM postgres_scan('', 'public', 'my_table');
```