Transforms to allow group by geo_point

[thekofimensah]

I'm creating an entity centric index using transforms and I'm finding that I can't group_by geo_point data type. This is too bad because I'm creating a profile for each of my local ip addresses that group by a couple of fields into the entity and I think it's bizarre I can't do that with a geo_point field.. should I be able to add it like any other field?

Currently I'm working around it by adding a pipeline to the transform but ideally I can do it directly in the transform.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)

[benwtrent]

Composite aggs support group_by geotile. So, transforms COULD do something similar. But, the group_by items would be aggregated geotile buckets #45810