Feature request

Feature description

There are some users of the Parameter API in ROS1 that depend on getting a map of values, e.g. https://github.com/ros-teleop/teleop_twist_joy/blob/indigo-devel/src/teleop_twist_joy.cpp#L78 . It would be great to have a helper API to do this in ROS2 as well, to reduce the amount of boilerplate that users would have to do to get parameters one at a time.