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

[achou11]

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)

Tasks

• Add route handler for the stable style.json url implemented in Expose a method for retrieving a stable map style.json url #438, which does the described resolution strategy