s3manager.Upload API shape — align with upcoming Download manager

s3manager.Upload API shape — align with upcoming Download manager: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I'm working on adding a download manager to s3manager (counterpart to the existing upload manager). While sketching the API I realized the two are…