vfs: --vfs-used-is-size to report used space using recursive scan (#4043)

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>

vfs/vfscommon/options.go

@@ -32,6 +32,7 @@ type Options struct {
 	ReadWait          time.Duration // time to wait for in-sequence read
 	WriteBack         time.Duration // time to wait before writing back dirty files
 	ReadAhead         fs.SizeSuffix // bytes to read ahead in cache mode "full"
+	UsedIsSize        bool          // if true, use the `rclone size` algorithm for Used size
 }
 
 // DefaultOpt is the default values uses for Opt
@@ -58,4 +59,5 @@ var DefaultOpt = Options{
 	ReadWait:          20 * time.Millisecond,
 	WriteBack:         5 * time.Second,
 	ReadAhead:         0 * fs.MebiByte,
+	UsedIsSize:        false,
 }