add path prefix to ObjectStorage.Iterator (#23332)

Support to iterator subdirectory in ObjectStorage for
ObjectStorage.Iterator method.

It's required for https://github.com/go-gitea/gitea/pull/22738 to make
artifact files cleanable.

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/storage/helper.go

@@ -90,6 +90,6 @@ func (s discardStorage) URL(_, _ string) (*url.URL, error) {
 	return nil, fmt.Errorf("%s", s)
 }
 
-func (s discardStorage) IterateObjects(_ func(string, Object) error) error {
+func (s discardStorage) IterateObjects(_ string, _ func(string, Object) error) error {
 	return fmt.Errorf("%s", s)
 }