FeatureInfo: Support parsing values from WMS raster sources that lack both ID and geometry

[jacobwod]

Sometimes we want to read values from sources that lack typical information we usually get from vector-based sources. One example is a GeoTIFF raster where each color is mapped to a value (e.g. height above sea level, or temperature).

If those layers are queried using GeoJSON, there's no way to tell which feature belongs to which layer (because the returned features lack IDs).

But if we request them as GML or text/xml, we might get something that we can work on:

<?xml version="1.0" encoding="UTF-8"?>
<wfs:FeatureCollection xmlns="http://www.opengis.net/wfs" xmlns:wfs="http://www.opengis.net/wfs" xmlns:gml="http://www.opengis.net/gml" xmlns:bmf="https://bmf.halmstad.se" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/wfs https://karta.halmstad.se/geoserver/schemas/wfs/1.0.0/WFS-basic.xsd">
    <gml:boundedBy>
        <gml:null>unknown</gml:null>
    </gml:boundedBy>
    <gml:featureMember>
        <bmf:ndem_2022 fid="">
            <bmf:GRAY_INDEX>7.832002639770508</bmf:GRAY_INDEX>
        </bmf:ndem_2022>
    </gml:featureMember>
    <gml:featureMember>
        <bmf:dtm_2022 fid="">
            <bmf:GRAY_INDEX>24.24799919128418</bmf:GRAY_INDEX>
        </bmf:dtm_2022>
    </gml:featureMember>
</wfs:FeatureCollection>

The example above contains both one attribute value (GRAY_INDEX) and information on WMS layer name in the response, making it possible for us to match the infoclick definition with the correct layer.

Unfortunately, OpenLayers' built-in parsers fail on generating usable feature IDs (FIDs) here. Hence, a custom solution is needed that will take care of this and produce something really usable.

(This is remotely related to #1099, #1258 and #1266.)