[ML] Better support for geo in datafeeds

[benwtrent]

We should allow reading from a geo_point piece of data when using the lat_long detector. The Datafeed needs to automatically account for either a raw geo_point or a geo_centroid (in the event of aggregated data) data point. The formatting (comma delimited string) should happen automatically in the Datafeed before it sends the data back to the ML process.

In short, for the lat_long detector, a Datafeed should support data from: geo_point field, geo_centroid agg, geo_shape field (point type only), and the legacy (current) scripted field option.

This will require a fair bit of work within the extractor logic and some within the validation logic.

[elasticmachine]

Pinging @elastic/ml-core