load_collection: proper geometry spatial_extent support

[soxofaan]

(reported by @VictorVerhaert / @VincentVerelst)
load_collection supports GeoJSON/Vector cubes as spatial_extent and the spec mentions that this should involve masking (in the sense of mask_polygon/filter_spatial):

For raster data, all pixels inside the bounding box that do not intersect with any of the polygons will be set to no data (null).

Experiments of @VictorVerhaert however show that the geopyspark implementation just takes the bounding box of the geometries and does not apply masking.

Side-observation: the geopyspark implementation also supports feature collections containing only points (in the sense that the total bounding box is used), while the spec does not list point support. Related issue: Open-EO/openeo-processes#502