Support `ORDER BY ... NULLS FIRST/LAST` in the SQL API

**Data model**
Consider the following data model:
```yaml
cubes:
- name: employees
sql: >
SELECT 1 AS id, 'Ali' AS first_name, 'Los Gatos' AS city UNION ALL
SELECT 2 AS id, 'Bob' AS first_name, 'San Diego' AS city UNION ALL
SELECT 3 AS id, 'Eve' AS first_name, 'San Diego' AS city UNION ALL
SELECT 4 AS id, 'Foe' AS first_name, NULL AS city

measures:
- name: count
type: count

dimensions:
- name: city
sql: city
type: string
```

**SQL queries**
I would like the SQL API to support `ORDER BY ... NULLS FIRST/LAST`, similarly to how Postgres supports it: https://www.postgresql.org/docs/current/queries-order.html

Currently, the SQL API just ignores `NULLS FIRST/LAST`:
```bash
# CUBESQL_SQL_PUSH_DOWN=false
=> SELECT city, count FROM employees ORDER BY city;
city | count
-----------+-------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)

=> SELECT city, count FROM employees ORDER BY city NULLS FIRST;
city | count
-----------+-------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)

=> SELECT city, count FROM employees ORDER BY city NULLS LAST;
city | count
-----------+-------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)

# CUBESQL_SQL_PUSH_DOWN=true
=> SELECT city, MEASURE(count) FROM employees GROUP BY city ORDER BY city;
city | measure(employees.count)
-----------+--------------------------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)

=> SELECT city, MEASURE(count) FROM employees GROUP BY city ORDER BY city NULLS FIRST;
city | measure(employees.count)
-----------+--------------------------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)

=> SELECT city, MEASURE(count) FROM employees GROUP BY city ORDER BY city NULLS LAST;
city | measure(employees.count)
-----------+--------------------------
Los Gatos | 1
San Diego | 2
| 1
(3 rows)
```

I would expect the results of the second query to have the row with `NULL` in `city` at the first position.

**Version:**
0.35.58

**Additional context**
Inspired by a conversation in Slack: https://cube-js.slack.com/archives/C04NYBJP7RQ/p1720814502553919