feat(bindings/cpp): make ReaderStream manage the lifetime of Reader

[silver-ymz]

Original API seperates the lifetime of Reader and ReaderStream. Users cannot easily manage them. For example, following code will be wrong previously because of stack-use-after-scope.

opendal::ReaderStream return_stream(opendal::Operator &op, std::string_view path) {
  return op.reader(path);
}

With this update, ReaderStream will take the ownership of Reader. Users can ignore Reader when using ReaderStream.