S3Guard CLI to support list/purge of pending multipart commits

The S3A CLI will need to be able to list and delete pending multipart commits.

We can do the cleanup already via fs.s3a properties. The CLI will let scripts stat for outstanding data (have a different exit code) and permit batch jobs to explicitly trigger cleanups.

This will become critical with the multipart committer, as there's a significantly higher likelihood of commits remaining outstanding.

We may also want to be able to enumerate/cancel all pending commits in the FS tree