Support ORDER BY within GROUP_CONCAT

[tbantle22]

This is used by the dolt workbench

Example query:

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

[tbantle22]

Related: #793