expose MAPBOX_GEOCODER_API_KEY

[paullryan]

So that we can provide the MAPBOX_GEOCODER_API_KEY from a loaded service (e.g. get the key from server side configs). We should expose MAPBOX_GEOCODER_API_KEY like we do with MAPBOX_API_KEY.

Allows for

    {
      provide: MAPBOX_GEOCODER_API_KEY,
      useFactory: mapboxKeyFactory,
      deps: [EnvService]
    },

where mapboxKeyFactory looks something like

export function mapboxKeyFactory(env: EnvService) {
  return env.mapbox.accessToken;
}

and the EnvService loads the env from an external source.