Feature request

Feature description

Using the action client in rclcpp_action requires polling using timers on the future objects returned by Client::async_send_goal and Client::async_get_result. It would be far more convenient to add callbacks to these functions.

(spin_until_future_complete won't work because my node is already spinning.)

Implementation considerations

Possible function signatures could look like this:

using GoalResultCallback =
  std::function<void(std::shared_future<typename GoalHandle::SharedPtr>)>;
using ResultCallback =
  std::function<void(std::shared_future<Result>)>;

std::shared_future<typename GoalHandle::SharedPtr>
async_send_goal(
  const Goal & goal, GoalResultCallback = nullptr,
  FeedbackCallback callback = nullptr, ResultCallback = nullptr,
  bool ignore_result = false);

std::shared_future<Result>
async_get_result(typename GoalHandle::SharedPtr goal_handle, ResultCallback = nullptr)

While these are the callbacks I currently need, it would be consistent to add appropriate callbacks to all functions returning future objects.