`ORDER BY` support in aggregate functions

[fulghum]

Postgres aggregate functions (e.g. string_agg()) can take an optional ORDER BY clause as the last argument. Doltgres doesn't support this syntax yet.

Example query:

SELECT type.typname AS name, type.OID AS oid, n.nspname AS schema, string_agg(enum.enumlabel, ',' ORDER BY enum.enumsortorder) AS value
FROM pg_enum AS enum
JOIN pg_type AS type ON (type.oid = enum.enumtypid)
JOIN pg_namespace n ON type.typnamespace = n.oid
WHERE n.nspname = ANY (current_schemas(false))
GROUP BY type.OID, n.nspname, type.typname;

Found while testing a simple Rails app with Doltgres.

[fulghum]

Confirmed this was fixed in #1515