Add support for GROUP BY

[jewzaam]

Currently stubbed out and not supported. Will also require some support for aggregate functions, so non-group-by columns can be returned in aggregate (min, max, sum, count, etc).

[luan-cestari]

Could you give more details how do you think the implementation would be? Specially the input from the lightblue

[jewzaam]

The simplest use case I can think of that we need is ability to do distinct on a column and return that to a client. But that isn't this specific issue, so I'd just leave it in backlog until there's a clear use case defined.

[luan-cestari]

Moved to backlog

[jewzaam]

@jblashka fyi