[Maps] Seperate Maps and Map Labels

[Kolano]

Describe the feature:
The current default road_map includes region borders and map labels on the same layer. This results in maps labels being obscured by any added layer. It would be nice if map labels were in a separate layer, allowing them to appropriately overlay content.

This would...

• Improve the legibility of most map displays
• Avoid odd multicolored text for labels that cover multiple regions / water

Describe a specific use case for the feature:
Legibility of the Map visualization would be improved by this across use cases.

[elasticmachine]

Pinging @elastic/kibana-gis

[nickpeihl]

This may be possible when we enable support for vector tiles in the Maps plugin.