Added `addAggs()` method Query. Fixes #4451

fixes #4451
I used `aggs` because after reading elasticsearch reference guide, I got an impression that the way to go is `aggs` instead of `aggregations`, a lot of examples in reference guide uses `aggs` maybe thats why.

form the reference guide:
> The aggregations object (the key aggs can also be used) in the JSON holds the aggregations to be computed

and I also like that `addAggs()` is shorter than `addAggregations()`, but  `aggregations` would be more readable.

Should I change it to `aggregations` ?