Revise add wms/wfs data source handling

This is the WMS and WFS for the AWI hosted data:

WMS: http://maps.awi.de/services/projects/digital_earth/mueglitz_tal/wms?service=WMS&request=GetCapabilities&version=1.3.0

WFS: http://maps.awi.de/services/projects/digital_earth/mueglitz_tal/wfs?service=WFS&request=GetCapabilities&version=2.0.0

Please test and revise the WMS/WFS data source handling code accordingly.

I took a look myself but realized, that there are multiple variations and places (e.g. DatasourceGetter, DemapAddLayerForm, WFSDescribeFeatureTypeConverter) where the service capabilities are requested and parsed. This is not very maintainable, please revise and place the requests from one class and in one flavor only. Also consider the CORS handling. 'no-cors' mode is not what we want. If there is a CORS problem use the ReverseProxy, either via 'axios' or by manually prepending the PROXY_URL and passing the header field as in '~/plugins/axiosCorsInterceptor'.