add payloads to Action

[facontidavide]

The main goal of this PR is to support users that want to send and receive more comple payloads, when executing a Tree (arguments and return values).

A single string is used for both, since the majority of the users can use JSON to store any argitrary set of arguments.

The return value of onTreeExecutionCompleted has been changed to support a return value.

Also, about this code :

  // return success or aborted for the action result
  if(status == BT::NodeStatus::SUCCESS)
  {
    RCLCPP_INFO(kLogger, "BT finished with status: %s", BT::toStr(status).c_str());
    goal_handle->succeed(action_result);
  }
  else
  {
    action_result->error_message = std::string("Behavior Tree failed during execution "
                                               "with status: ") +
                                   BT::toStr(status);
    RCLCPP_ERROR(kLogger, action_result->error_message.c_str());
    goal_handle->abort(action_result);
  }

I don't think that a FAILURe shoult trigger a goal_handle->abort since, from a technical point of view, the tree has been executed.

[facontidavide]

@MarqRazz check my latest changes