Function MakeGeneratorStartsWith

cpp/src/arrow/util/async_generator.h:1997–2003

/// pull from following_values async-reentrantly.
template <typename T>
AsyncGenerator<T> MakeGeneratorStartsWith(std::vector<T> initial_values,
                                          AsyncGenerator<T> following_values) {
  auto initial_values_vec_gen = MakeVectorGenerator(std::move(initial_values));
  auto gen_gen = MakeVectorGenerator<AsyncGenerator<T>>(
      {std::move(initial_values_vec_gen), std::move(following_values)});
  return MakeConcatenatedGenerator(std::move(gen_gen));
}

template <typename T>
struct CancellableGenerator {