[Feature Request] Postgres DISTINCT ON support

[FlightOfStairs]

We would like to be able to express Postgres' SELECT DISTINCT ON construct in the ktorm DSL.

Example use-case is below (fiddle):

CREATE TABLE users (id VARCHAR, version BIGINT, data VARCHAR);

INSERT INTO users (id, version, data) VALUES
  ('id-1', 1, 'foo'),
  ('id-1', 2, 'bar'),
  ('id-2', 1, 'alpha'),
  ('id-2', 2, 'beta'),
  ('id-2', 3, 'gamma');

SELECT DISTINCT ON (id) id, data FROM users ORDER BY id, version DESC;

Results should be the latest for each user, i.e.,

id-1,bar
id-2,gamma

Currently we are doing two round trips to first fetch the set of latest versions, then fetch only those versions. Subquery support would also allow a single trip.