Add geometry metadata in `gpf_wfs_get_features` response

[esgn]

• Geometry surface (when polygon)
• Geometry length (when linear)
• Distance to spatial filter reference (geom to geom distance)
• Intersection area with spatial filter reference

[LionelZoubritzky-IGN]

Could be added as options to geometry_keep if the design of #155 is accepted.

In that case, the geometry could be replaced by a feature field, being a GeoJSON.Feature object including different aspects of the returned object (centroid as geometry, bbox, surface, length, distance to spatial filter, etc.). We could also include a description field to that feature to explain to the LLM what is stored in the resulting object, that could be helpful.