Separate part sizes for GET and PUT

[jamesbornholt]

Tell us more about this new feature.

Sometimes we might want to use different part sizes for uploads vs downloads, for cost or performance reasons. For example, you might want a large PUT part size to support large file uploads, but a smaller GET part size for maximum throughput. Today they're configured with the same --part-size argument. It would be nice if they could be configured separately.