Support limiting sub-groups to top N

In Elasticsearch we can do a group by gender, and then sub-group in each gender by state, and sum a metric, and get the top 10 results (for that sum of that metric) per state.

There doesn't seem to be a way to currently do that in Pinot, though @kishoreg suggested it would be possible via a UDF (let's call it "subBuckets"), something like `select , subBuckets(, , ) from group by `. For example:

``` sql
select gender, subBuckets(sum(billed_expenses), state, 10) from medicare_expenses group by gender
```

The issue names no files, tests, or entry points. Start by examining Pinot's support for grouped aggregations and the proposed subBuckets SQL shape; done means the example query can return the top 10 metric results within each gender and state grouping.

Written by the indexing model from the issue text.