Support for `any_value` aggregation function

Many databases provide an `any_value` aggregation function, which is useful in queries like this:

```sql
select colA, any_value(colB)
from T
group by colA
```

Start with the SQL example and inspect Pinot's existing aggregation-function support and related tests. Done means a query using any_value(colB) with GROUP BY colA executes successfully and returns an appropriate value for each group.

Written by the indexing model from the issue text.