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

[igorlukanin]

Data model Consider the following data model:

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:

# 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