[New Feature]: Initiators for RDRgen processing

[LucaCinquini]

Implement triggering of RDRgen execution.
This could be based on the Unity Initiatiors framework (see https://github.com/unity-sds/unity-initiator), triggered by the output of EDRgen being stored on S3; or it could be based on the EDRgen DAG triggering direct execution of the RDRgen DAG.