[Maps] Document layers should support heatmap styling

[thomasneirynck]

Individual documents can only be styled using the vector-style properties (fill-color, icon-size, ...).

This layer should also support a heatmap-style. Right now, heatmaps are only supported as a separate layer-type "heatmap layer". This layer-type uses an aggregation in the background to avoid retrieving all the documents.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

Can you provide some background on the intended use case and where this request came from?

[thomasneirynck]

This would be useful in a few scenarios:
- support styling for timeslidered-data, which will likely introduce client-side filtering based on timestamp. heatmaps for moving tracks is fairly common
- useful for shapes, e.g. lines, where users wants to blend track-paths together (e.g. the tracks-layer)
- it would be useful when the 10k limit starts to matter less, e.g. when .mvt generation can pack together hundreds to millions of document location in a tile (elastic/elasticsearch#58696)

[nreese]

Closing, heatmap layer has added support for high resolution gridding and this provides a much better heatmap.

Having heatmaps in multiple places, documents layer and aggregation layer, would be a UX nightmare of explaining how,why, when to use. Only having heatmap layer for aggregations keeps everything simple.