Push down subqueries on reference tables when result is deterministic

We currently do not push down subqueries that do not pass the checks in `DeferErrorIfCannotPushdownSubquery`, even if the subquery contains no distributed tables.

For subqueries with deterministic result sets across all workers, we could push them down, while for non-deterministic results we should use recursive planning.

Examples of deterministic results:
```sql
SELECT * FROM reference_table ORDER BY x LIMIT 10
SELECT max(x) FROM reference_table
SELECT * FROM reference_table UNION SELECT * FROM other_reference_table
```

Examples of non-deterministic results:
```sql
SELECT * FROM reference_table LIMIT 10
SELECT DISTINCT ON (x), y FROM reference_table
SELECT * FROM reference_table WHERE x < random()
```

related to #1872