feat: Fetch style from url

[YoelRidgway]

This PR adds the ability to fetch remote JSON stylesheets by specifying a url in the config.

For example:

"styles": {
    "streets-v2": {
      "style": "https://api.maptiler.com/maps/streets-v2/style.json?key=********************",
      "tilejson": {
        "type": "overlay"
      }
    },
    "maplibre-demotiles": {
       "style": "https://demotiles.maplibre.org/style.json"
    }
}

Resolves #1460