add subdomain support

[johnpryan]

This is a test branch that enables CORS support. I still need to do some testing to see if this improves performance for various map providers

to use, you can put the {s} tag in your URL template and supply the subdomains:

layers: [
  new TileLayerOptions(
    urlTemplate:
        "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png",
    subdomains: ['a','b','c']
  ),
  new MarkerLayerOptions(markers: markers)
],

FYI @wmleler