Support `ORDER BY` within `GROUP_CONCAT`

This is used by the dolt workbench

Example query:
```sql
SELECT
table_name, index_name, comment, non_unique, GROUP_CONCAT(column_name ORDER BY seq_in_index) AS COLUMNS
FROM information_schema.statistics
WHERE table_catalog='doltgres/main' AND table_schema='public' AND table_name='test' AND index_name!='PRIMARY'
GROUP BY index_name;
```

Skipped test [here](https://github.com/dolthub/doltgresql/blob/main/testing/postgres-client-tests/node/workbenchTests/table.js#L147)