Introduces getGetLegendGraphic method

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

examples/wms-getlegendgraphic.html

@@ -0,0 +1,15 @@
+---
+layout: example.html
+title: WMS GetLegendGraphic
+shortdesc: Example of a WMS GetLegendGraphic.
+docs: >
+  WMS supports the [getLegendGraphic request](https://docs.geoserver.org/latest/en/user/services/wms/get_legend_graphic/index.html).
+  This example demonstrates the use of the `getGetLegendGraphicUrl` method.
+
+
+  As a legend can be responsive to the scale it is updated on every change of the resolution.
+tags: "GetLegendGraphic, getGetLegendGraphicURL, WMS"
+---
+<div id="map" class="map"></div>
+Legend:
+<div><img id="legend" src=""/></div>