Parametrize prediction keys

[geohacker]

Prediction data is stored as JSONB. This means we can store any arbitrary key that's produced by the model. At the moment, ml-enabler API is built to aggregate two keys - ml_prediction and osm_building_area, because both the predictors are dealing with building data. The ml-enabler-cli, will produce the payload that can be readily submitted to the api.

In the future, when a new predictor is added, the cli can appropriately set the keys. At that time, the API would have to do one or more of the following:

1. Accept keys as query params to aggregate
2. Store keys that can be aggregated in the Prediction table -- so it doesn't confuse with things like URLs or other string values