Add support for limit pushdown through union

[fgwang7w]

== RELEASE NOTES ==
General Changes
* Add support for limit pushdown through union

resolve #14894

[wenleix]

LGTM.

[kaikalur]

Will be nice if this applies to ORDER BY LIMIT as well.

LGTM

[rongrong]

So, theoretically, this won't fire if the limit here is > 1 either right? Maybe also test that?

[fgwang7w]

sure I add another test to cover limit2 case

[rongrong]

I prefer a separate check before building the list. Something like

if (unionNode.getSources().stream().allMatch(source -> isAtMost(source, context.getLookup(), parent.getCount()))) {
    return Result.empty();
}
// build new plan node

It's slightly more efficient when the rule should not fire, which should happen more often.

[fgwang7w]

yes this check makes sense, will incorporate it.

[rongrong]

You don't need to check shouldApply anymore after the check. It should always apply if the code gets here.