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_test.go

@@ -42,7 +42,7 @@ func Test_discardStorage(t *testing.T) {
 				assert.Errorf(t, err, string(tt))
 			}
 			{
-				err := tt.IterateObjects(func(_ string, _ Object) error { return nil })
+				err := tt.IterateObjects("", func(_ string, _ Object) error { return nil })
 				assert.Error(t, err, string(tt))
 			}
 		})