Implement style.json resolution strategy a when handling the stable style url #2

[ximenabb]

linked to Implement style.json resolution strategy a when handling the stable style url #439

Description
A frontend client is provided a single endpoint to use for retrieving a style.json definition (see #438). This endpoint should be handled by the backend, which performs the following resolution strategy to determine the style.json payload that's returned as the response:

1-Check if there's a offline static map with the style id of default (or whatever name we decide on). In practice, this means that there's a file that exists on the filesystem at ROOT/default/style.json, where ROOT is a provided directory where static custom maps are stored. If it exists, serve the style.json content as the response

2- if internet access is available, proxy a default online style of our choosing (e.g. using a mapbox style that requires our account's public access token)
3- if internet access is not available, serve an fallback map style that uses statically served local geojson files (e.g. https://github.com/digidem/mapeo-offline-map)

##Todo

• if internet access is available, proxy a default online style of our choosing (e.g. using a mapbox style that requires our account's public access token)

[achou11]

@ximenabb @EvanHahn I decided to spend time addressing this on my own since I was able to get most of the implementation done within a reasonable amount of time: #459

Feel free to either close this issue or let the PR close it when merged

[achou11]

Think it makes sense to close this in favor of #459 . Confusing to have this here