"context" argument support for "callback as a callable" feature

[soxofaan]

related to #223 :

The "callback as a callable" feature (https://open-eo.github.io/openeo-python-client/processes.html#callback-as-a-callable) only support arguments like "x" or "data", e.g.

def avg(data):
    return data.mean()

cube.reduce_dimension(avg, dimension="t")

These "child processes" in principle can also access a "context" argument, so support should be added for that in some way.