fs: add Optional ID() method to Object and implement it in backends

ID() shows the internal ID of the Object if available.

backend/box/box.go

@@ -1048,6 +1048,11 @@ func (o *Object) Remove() error {
 	return o.fs.deleteObject(o.id)
 }
 
+// ID returns the ID of the Object if known, or "" if not
+func (o *Object) ID() string {
+	return o.id
+}
+
 // Check the interfaces are satisfied
 var (
 	_ fs.Fs              = (*Fs)(nil)
@@ -1058,4 +1063,5 @@ var (
 	_ fs.DirMover        = (*Fs)(nil)
 	_ fs.DirCacheFlusher = (*Fs)(nil)
 	_ fs.Object          = (*Object)(nil)
+	_ fs.IDer            = (*Object)(nil)
 )