Add WFS support

[kannes]

@pathmapper and I were trying spatial queries on WFS layers and it might "just work". Not sure if every WFS will allow for it but it might be worth trying.

Example:
On https://www.wfs.nrw.de/geobasis/wfs_nw_dvg -> dvg:nw_dvg2_krs the filter SELECT * FROM nw_dvg1_krs WHERE ST_Intersects(msGeometry, ST_GeometryFromText('POINT(455600 5720000)')) works.

msGeometry is the name of the geometry column. We looked it up manually on https://www.wfs.nrw.de/geobasis/wfs_nw_dvg?SERVICE=WFS&REQUEST=DescribeFeatureType&VERSION=2.0.0&TYPENAMES=dvg:nw_dvg2_krs&TYPENAME=dvg:nw_dvg2_krs

It might be ugly/hard to get this via PyQGIS as layer.dataProvider().uri().geometryColumn() returns an empty string :(

Being able to filter (slow and quickly overloaded) WFS servers would be great!

[kannes]

The storage type is 'OGC WFS (Web Feature Service)'

OGC API Features would be useful too.

[kannes]

We need to check if the filter actually results in a filtering WFS query (server provides filtered data) or if it is just client-side filtering (full data (in bbox) is downloaded).

Does it work?
Does it work for WFS1, WFS2 and OAPIF?