/// Returns a list of valid configuration keys in `keys` array and number of
  /// keys in `num_keys`. Ownership of the buffers in `keys` are transferred to
  /// caller and needs to be freed using relevant filesystem allocation API.
  ///
  /// Plugins:
  ///   * Must set `status` to `TF_OK` on success. If there are no configurable
  ///     keys, `num_keys` should be set to 0