Support `default_job_options` from custom predefined processes

[soxofaan]

#366 added support for default_job_options defined in real UDPs or remote process definitions.

https://github.com/eu-cdse/openeo-cdse-infra/issues/1034 requests extending that to predefined processes that are defined through a JSON process graph too (aka "custom" processes in the Terrascope deployment)