Both geo_bounding_box query and geo_bounds aggregation have
a very similar definition of a "bounding box". A lot of this
logic (serialization, xcontent-parsing, etc) can be centralized
instead of having separated efforts to do the same things

It is fairly common to filter the geo point candidates in
geohash_grid and geotile_grid aggregations according to some
viewable bounding box. This change introduces the option of
specifying this filter directly in the tiling aggregation.