Support top-level run_udf in validation mode (like in wet run)

[soxofaan]

#364 added a validation error item from top-level run_udf usage:

openeo-python-driver/openeo_driver/ProcessGraphDeserializer.py

Lines 1792 to 1793 in 21e3890

	if env.get("validation", False):
	raise FeatureUnsupportedException("run_udf is not supported in validation mode.")

which was ok at the time as the python client only shows this as a warning, but still allows user to submit the job.

Since Open-EO/openeo-web-editor#309 (comment) however, the web editor started blocking workflows when pre-flight validation returns errors.

This makes it impossible to submit jobs involving top-level run_udf nodes through web editor