petascope - legend for WMS styles

[Bang Pham Huu]

For each WMS style, we need to allow the admin to define a legend url (type string). The legend url should be stored in the layer definition, under:

 <element ref="wms:LegendURL" minOccurs="0" maxOccurs="unbounded"/>

example:

<Style>
	<Name>osm:osm_water_areas</Name>
	<Title>osm:osm_water_areas</Title>
	<LegendURL width="22" height="120">
           <Format>image/png</Format>
   	   <OnlineResource
	       xmlns:xlink="http://www.w3.org/1999/xlink" 
               xlink:type="simple" 
               xlink:href="https://ahocevar.com/geoserver/ows?service=WMS&request=GetLegendGraphic&format=image%2Fpng&width=20&height=20&layer=osm%3Awater_areas"/>
         </LegendURL>
</Style>

• admin can provide text as legend url (could be an url or base64 encoded binary image data)
• the provided legend url should be available in the layer definition
• add support for GetLegendGraphic request (only for request, layer and style parameters), which would simply return the legend url if one is defined for the style, example: ​https://ahocevar.com/geoserver/ows?service=WMS&request=GetLegendGraphic&format=image%2Fpng&width=20&height=20&layer=osm%3Awater_areas