Loading of additional map layers via config

[spwoodcock]

I think it makes total sense to replace the frontend prototype for the new OAM with stac-map: hotosm/openaerialmap#234

Prerequisite: #136

• It would be great if we can also load additional layers as part of a config.
• I would imagine we would load a global coverage layer (PMTiles), or a TMS layer (TiTiler) into the map as needed.

e.g. config.json

{
  "stac-url": "https://xxxx",
  "layers": [
    'mapbox style spec layers here'
    'pmtiles, tms, etc'
  ]
}