OL: WMS tile layer support for map style switcher

[jsmith-informed]

Adds wms as a map style type for the OpenLayers provider, required so we can add the APGB aerial base map.

e.g

{
  id: 'apgb-aerial-125mm',
  label: 'Aerial 12.5cm',
  type: 'wms',
  url: 'https://www.getmapping.com/GmWMS/{YOUR-UUID}/ApgbBng.wmsx'
  params: { LAYERS: 'APGB_Latest_UK_125mm', BGCOLOR: '0x1E3448', TRANSPARENT: false },
}

Also added a check on the type to ensure its valid to alert to config mistakes.

Added to the demos and tested there: