add support for FOR ORDINALITY in JSON_TABLE

[lovasoa]

Initially reported in sqlpage/SQLPage#658

This is a follow-up on #1019

I already implemented most of json_table, but we are still missing support for FOR ORDINALITY columns.

Reference: https://dev.mysql.com/doc/refman/8.0/en/json-table-functions.html#function_json-table

Exemple:

SELECT jt.*
FROM JSON_TABLE(
  '["Alice", "Bob", "Charlie"]',
  '$[*]' COLUMNS(
    row_num FOR ORDINALITY,
    name VARCHAR(50) PATH '$'
  )
) AS jt;

try online