Add support for crossOrigin option

[ghost]

Cross origin is required for forEachLayerAtPixel method, for example. Without that option user gets an error:

intermediatecanvas.js:140 Uncaught DOMException: Failed to execute 'getImageData' on 'CanvasRenderingContext2D': The canvas has been tainted by cross-origin data.
    at ol.renderer.canvas.TileLayer.ol.renderer.canvas.IntermediateCanvas.forEachLayerAtCoordinate (http://localhost:8180/build/ol/renderer/canvas/intermediatecanvas.js:140:44)
    at ol.renderer.canvas.Map.forEachLayerAtPixel (http://localhost:8180/build/ol/renderer/canvas/map.js:207:30)
    at ol.Map.forEachLayerAtPixel (http://localhost:8180/build/ol/map.js:627:25)
    at ol.Map.hoverUnlistenKey_.map.on (http://localhost:8180/common/map/click.js:159:11)
    at ol.Map.boundListener (http://localhost:8180/build/ol/events.js:17:21)
    at ol.Map.ol.events.EventTarget.dispatchEvent (http://localhost:8180/build/ol/events/eventtarget.js:88:24)
    at ol.Map.handleMapBrowserEvent (http://localhost:8180/build/ol/map.js:944:12)
    at ol.MapBrowserEventHandler.boundListener (http://localhost:8180/build/ol/events.js:17:21)
    at ol.MapBrowserEventHandler.ol.events.EventTarget.dispatchEvent (http://localhost:8180/build/ol/events/eventtarget.js:88:24)
    at ol.MapBrowserEventHandler.relayEvent_ (http://localhost:8180/build/ol/mapbrowsereventhandler.js:280:8)

That exception is occurred because of CORS.
Link to OpenLayers doc: https://openlayers.org/en/latest/apidoc/ol.source.WMTS.html