feat(query): add a planner rule to push down bare aggregates

[wolffcm]

This adds a rule to push down aggregates that are directly over a ReadRange source.

I followed the existing example of PushDownWindowAggregateRule and did some refactoring to avoid duplicating logic.

I'll follow up with a pipeline test and a Flux end-to-end test.

[adrian-thurston]

Looks good. Nice to be able to reuse the window aggregate physical node.

Note the feature flags descriptions are written to indicate they are enabling the rewrite rule, not the pushed down physical node. But with this change the feature flags will cover both window aggregate and bare aggregate rules. That's fine with me. Others might want them separate. If not, we just needs to make it clear the flags cover multiple rules.

[wolffcm]

@adrian-thurston

So with this change the feature flags will cover both window aggregate and bare aggregate rules.

Hmm, I hadn't really thought that through. I was thinking of the feature flag as guarding the storage capability, but I guess that's not the case. I guess the question comes down to: is it possible that we introduce a regression with the bare-agg rule but not the window-agg rule? It seems within the realm of possibility.

I would think that we'd want to avoid an explosion of feature flags though. I'm curious about @yzhang1991 thoughts on this.

[ethanyzhang]

LGTM