support offline map data that can be shared by extensions

[ES-Alexander]

Likely multiple extensions will want to use map data (e.g. Cockpit, DVL for setting start position, etc), and it seems a shame for each service to need to download and store the map data for the same area, so it could be useful if BlueOS provides a service that can download and store map tiles, and provide them to any extensions that need to display a map.

This is most useful for vehicles with a high bandwidth connection (e.g. it's more useful for subs than boats)

[ES-Alexander]

Relevant to bluerobotics/BlueOS-Water-Linked-DVL#28