feat: lunchpail enqueue commands

[starpit]

• lunchpail enqueue file
• lunchpail enqueue s3
• lunchpail qdone -- for dispatchers to signal they have no more work to dispatch
• Ports ParameterSweep and ProcessS3Objects to use these features.
• Removes the ProcessS3Objects shell script. That dispatcher is now entirely go (though still with yaml wrappers that we should reevaluate).
• TODO: the ParameterSweep shell script remains, though it does nowuse lunchpail enqueue file.

This helps us remove a few lingering reliances on rclone. It should also help with implementing custom dispatchers, as they should no longer need to deal with rclone and queue path conventions directly.