[AGG-Rule]

[Jigao Luo]

Hi Calcite Contributors,

I have a question about Aggregation Enumerable Rules (Converter Rule). 
We know that EnumerableAggregate and EnumerableSortedAggregate are 
physical operators of a logical Aggregate operator. I would like to know 
if we have implemented rules or costs to select one of them when 
building a physical plan.

So this is what I mean:
```
planner.addRule(EnumerableRules.ENUMERABLE_AGGREGATE_RULE);
planner.addRule(EnumerableRules.ENUMERABLE_SORTED_AGGREGATE_RULE);
/// then in [Physical plan], one of them is selected depending on the query
```

Thanks!
Jigao