load_collection: support shapely geometry input

[jdries]

See this TODO:

openeo-python-client/openeo/rest/datacube.py

Line 193 in ced3faa

'spatial_extent': spatial_extent,

The official load_collection process allows geojson and vector cube as input, but it seems that the python client doesn't allow that.
This is very annoying, because it means we need tedious conversion to the 'spatial_extent' format everywhere.

Also a shapely bbox can be expressed rather easily as box(1.0,2.0,3.0,4.0), following the commonly used minx,miny,maxx,maxy notation that is found on tools like bbox finder.

There's already a helper function that might be of use:

openeo-python-client/openeo/rest/datacube.py

Line 1061 in ced3faa

def _get_geometry_argument(

[soxofaan]

just merged #682