support for arcgis mapserver

[lassitanskanen]

Hi,

I have to configure arcgis rest (both mapserver and feature service) to qwc2.

In ArcGIS Mapserver, it is possible request with multiple "layer" params.
(https://gis.stackexchange.com/questions/365765/merge-all-layers-become-a-single-layer-in-wms)

I suggest new layer type called "arcgis" that provide support for this kind of services. Another solution might be improve existing wms layer type to support "layers" param notation.

Here is external layer configuration suggestion:

  {
    "name": "mergedlayer",
    "type": "arcgis",
    "url": "https://localhost/arcgis/services/background_map/MapServer/WmsServer",
    "infoFormats": "application/json",
    "params": {
      "layers": "2,3,4"
    }
  },

Above should be generate as multiple layers params:
https://localhost/arcgis/services/background_map/MapServer/WmsServer?SERVICE=WMS&REQUEST=GetMap&layers=2&layers=3&layers=4