Add support for SELECT, UPDATE and DELETE statements against tables in
schemas.

Schema names are not allowed in Spanner SELECT statements. We need to
avoid generating SQL like

```sql
SELECT schema.tbl.id
FROM schema.tbl
```

To do so, we alias the table in order to produce SQL like:

```sql
SELECT tbl_1.id, tbl_1.col
FROM schema.tbl AS tbl_1
```

In sqlalchemy 2.0, it's:
```python
self._label_select_column(None, c, True, False, {}, include_table=False)
```

In older versions, it's
```python
self._label_select_column(None, c, True, False, {'include_table': False})
```

So instead, forward a flag to `vist_column` which can set this kwarg
safely itself.