[Postgres]: Structure of `orderby` AST does not match MySQL

Sorry for all the issues - having some trouble migrating to v5 as a lot of our Postgres tests are failing.

Parsing this query for MySQL and Postgres gives a different results structure. These were equivalent pre-v5 and both looked like the MySQL structure. This would be improved if the OrderBy type was not any:

SELECT col1 FROM tablename WHERE col1 = 'value' ORDER BY col2 DESC

MySQL:

{
   "orderby": [
        {
          "expr": {
            "type": "column_ref",
            "table": null,
            "column": "col2"
          },
          "type": "DESC"
        }
      ],
}

Postgres:

{
   "orderby": [
        {
          "expr": {
            "type": "column_ref",
            "table": null,
            "column": {
              "expr": {
                "type": "default",
                "value": "col2"
              }
            }
          },
          "type": "DESC",
          "nulls": null
        }
      ],
}