[Maps] Support mapbox vector style sheets

[thomasneirynck]

Support arbitrary mapbox vector style sheets (https://docs.mapbox.com/mapbox-gl-js/style-spec/) as a layer-type.

[thomasneirynck]

It's not entirely clear how this should be scoped:

• Add the style as a non-editable Kibana Maps layer. This would be similar to how EMS-styles are supported in Maps. This would be lower effort, but would really lock-in the saved-object configurations and Maps to the mapbox product suite (ie. mapbox-gl is the only library that implements all functionality of an arbitrary mapbox stylesheets)
• Add the style-sheet as a one or more editable Maps layers. This is challenging since it is unclear how mapbox layer styles should be mapped to a Kibana Maps layer. Would require support for [Maps] Group layers into tree view #37252

[thomasneirynck]

Let's close this. Nice2have, but no requests from outside.

Supporting TileJson may be more worthwhile (since user can always symbolize with the Kibana-layer-editor-UX)