Some backends, most notably S3, do not report the amount of bytes used.
This patch introduces a new flag that allows instead of relying on the
backend, use recursive scan similar to `rclone size` to compute the total
used space. However, this is ineffective and should be used as a last resort.
Co-authored-by: Yves G <theYinYeti@yalis.fr>
Before this change options were read and set in native format. This
means for example nanoseconds for durations or an integer for
enumerated types, which isn't very convenient for humans.
This change enables these types to be set with a string with the
syntax as used in the command line instead, so `"10s"` rather than
`10000000000` or `"DEBUG"` rather than `8` for log level.
