Expose a method for retrieving a stable map style.json url

[achou11]

Description

Mapping libraries often require a url that points to a map style definition, which defines how a map is rendered. In our case, we want a frontend client to be able to get a stable URL that points to the backend, which will do the work of figuring out which resource to send back (see #439).

This makes it much simpler for frontend integration, as it doesn't have to implement any logic to figure out which specific style resource to point to and would roughly look like this:

1. Retrieve the url e.g. const styleUrl = manager.getStyleJsonUrl()

2. Set that url for the map e.g. new mapboxgl.Map({ container: 'map' style: styleUrl }) (plain JS) or <MapView styleUrl={styleUrl}>... (React)

Tasks

• Implement a method on the MapeoManager for retrieving a stable style.json url.