text: Title: Missing .delete() for some SQL product classes
  Body: Add a basic .delete() implementation for `GenericSQLRelation` and `SQLRelation` to delete tables/views. I ran into an issue where I had to manually execute a `DROP TABLE` statement, but since metadata still marked the table as "up-to-date", it didn't execute, we should encourage users to use this method instead of dropping tables directly.
  
  However, we should also think how to prevent this. Maybe implement the `exists()` method? The main issue is that we'd have to execute a different query for some databases.