support set operation after pipeline

[wey-gu]

Now we could do this with Variable, while it's quite limited comparing to being allowed to be piped.

$var=LOOKUP ON player YIELD id(vertex) AS id
| GO FROM $-.id OVER follow REVERSELY
YIELD follow._dst AS event_id;
(
  GO from $var.event_id OVER serve YIELD $var.event_id, serve.start_year AS some_field
UNION ALL
  YIELD $var.event_id, NULL AS some_field
);