Feature request

Port topic_tools to ROS2 http://wiki.ros.org/topic_tools

Feature description

The topic_tools package provided a variety of useful utilities for manipulating topics:

• mux: multiplex between multiple topics.
• relay: republish data on one topic to another.
• relay_field: allow to republish data in a different message type
• drop: relay a topic, dropping X out of every Y message.
• throttle: relay a topic, but limit republishing to a maximum bandwidth or rate.
• transform: transform a topic (or topic field) using a Python expression and publishes the result on another topic.

These are likely to be quite useful to ros2 users as well.

Implementation considerations

Most of the functionality is likely straightforward, except for adaptive QoS, which can use the new get_topic_info RMW APIs being currently merged (same as the problem for rosbag2). However, the nodes could take in input and output QoS explicitly, as a early workaround if necessary.