Introduces getGetLegendGraphic method

Adds the `getGetLegendGraphic` method to `ImageWMS` and `TileWMS` source.
Also adds an example and corresponding tests.

examples/wms-getlegendgraphic.js

@@ -0,0 +1,47 @@
+import Map from '../src/ol/Map.js';
+import View from '../src/ol/View.js';
+import {Image as ImageLayer, Tile as TileLayer} from '../src/ol/layer.js';
+import ImageWMS from '../src/ol/source/ImageWMS.js';
+import OSM from '../src/ol/source/OSM.js';
+
+const wmsSource = new ImageWMS({
+  url: 'https://ahocevar.com/geoserver/wms',
+  params: {'LAYERS': 'topp:states'},
+  ratio: 1,
+  serverType: 'geoserver'
+});
+
+const updateLegend = function(resolution) {
+  const graphicUrl = wmsSource.getGetLegendGraphicUrl(resolution);
+  const img = document.getElementById('legend');
+  img.src = graphicUrl;
+};
+
+const layers = [
+  new TileLayer({
+    source: new OSM()
+  }),
+  new ImageLayer({
+    extent: [-13884991, 2870341, -7455066, 6338219],
+    source: wmsSource
+  })
+];
+
+const map = new Map({
+  layers: layers,
+  target: 'map',
+  view: new View({
+    center: [-10997148, 4569099],
+    zoom: 4
+  })
+});
+
+// Initial legend
+const resolution = map.getView().getResolution();
+updateLegend(resolution);
+
+// Update the legend when the resolution changes
+map.getView().on('change:resolution', function(event) {
+  const resolution = event.target.getResolution();
+  updateLegend(resolution);
+});