Support FAO GAUL WFS/WMS for non-GEE workflows

[dfguerrerom]

Summary

pygaul currently requires Google Earth Engine (GEE) for fetching geometries via pygaul.Items(). This proposal suggests adding native support for FAO's GAUL WFS/WMS services, enabling users to fetch administrative boundaries without GEE authentication:

• Some users might need admin boundaries but don't have or want GEE credentials
• Web apps and scripts could use pygaul without GEE infrastructure
• The package would be using official data
• pygaul already uses local parquet for names lookups - geometry fetching could follow the same pattern

FAO provides OGC-compliant services:

• WFS: https://data.apps.fao.org/map/gsrv/gsrv1/gaul/wfs
• WMS: https://data.apps.fao.org/map/gsrv/gsrv1/gaul/wms

We could have something like:

import httpx
import geopandas as gpd

params = {
    "service": "WFS",
    "version": "2.0.0", 
    "request": "GetFeature",
    "typeName": "gaul:gaul_2024_l0",
    "outputFormat": "application/json",
    "srsName": "EPSG:4326",
    "CQL_FILTER": "gaul0_code=57",
}

async with httpx.AsyncClient( as client:
    response = await client.get(FAO_WFS_URL, params=params)
    data = response.json()
    
gdf = gpd.GeoDataFrame.from_features(data["features"], crs="EPSG:4326")

Layers:

• Level 0: gaul:gaul_2024_l0
• Level 1: gaul:g2024_2023_1
• Level 2: gaul:g2024_2023_2

I'm implementing this in sepal-ui and it seems to works well. Happy to contribute a PR if there's interest.