[rosidl_generator_cpp] add <<operator for message classes

[mikaelarguedas]

We should provide a <<operator like in ROS 1 to ease printing to streams. Some places in the codebase print manually message content to stream in the meantime, e.g. https://github.com/ros2/demos/blob/144fb6d80810f149fa2457ae8542b332a6497674/demo_nodes_cpp/src/parameters/parameter_events.cpp#L26

[mikaelarguedas]

this will require a decision on what format (YAML, JSON, something else?) we want for the output.
Other PR related to the formats chosen: ros2/rclcpp#275

[dirk-thomas]

The decision from the last conversation on this topic was to not create such operators. Instead there should be global functions specialized on the message named to_yaml / to_json / to_whatever_you_like.

[wjwwood]

I didn't think that we ruled out an operator<< which defaulted to one of those, for convenience.