[jazzy] Add demux (backport #106)

[mergify[bot]]

Completes https://github.com/ros-tooling/topic_tools/issues/105

This PR porting https://github.com/ros/ros_comm/blob/noetic-devel/tools/topic_tools/src/demux.cpp to the ROS 2.

demux is a generic ROS topic demultiplexer: one input topic is fanned out to 1 of N output topics. A service is provided to select between the outputs

Design choices:

• Callbacks use lambda instead of std::bind to get default arguments working

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  This is an automatic backport of pull request #106 done by Mergify.