Allow choosing the multiprocessing start method for the Parallel executor

Allow choosing the multiprocessing start method for the Parallel executor: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). When I run a pipeline with the Parallel executor ( executor: parallel in pipeline.yaml ), Ploomber spawns child processes using whatever the…