✨ Add ESRI Rest support

[jbelien]

OpenLayers documentation: https://openlayers.org/en/latest/apidoc/module-ol_source_TileArcGISRest-TileArcGISRest.html

Example: http://geoportail.wallonie.be/catalogue/67777041-e399-4418-b9a4-38f621aab733.html