Support multiple topics via ros2 topic hz.

[fujitatomoya]

replaces #831 and closes #830

Note: after the merge, we need to update the documentation

[TannerGilbert]

Thanks for the quick reply on the other PR. I have one more question. Will changes like these only be merged into rolling or will there also be backports to older still supported versions like Humble?

[fujitatomoya]

@TannerGilbert

basically we enforce API compatibility for released distribution, see more details for https://docs.ros.org/en/rolling/The-ROS2-Project/Contributing/Developer-Guide.html#versioning. and this PR breaks API combability including behavior change, so we would not backport the fix to released distributions.

If we want to backport, what i would suggest is that,

1. merge this fix into rolling 1st, then wait for a couple of weeks as soak time to make sure nothing happens in the main.
2. and then, consider backport fix with API compatibility to jazzy, iron and humble. (each distro could have base code difference)

CC: @clalancette

[fujitatomoya]

@ahcorde @clalancette @sloretz i would like to request the review.

[fujitatomoya]

Pulls: #929
Gist: https://gist.githubusercontent.com/fujitatomoya/d0adaeee62fd2694795a2797a396d313/raw/ec0acd6ad03b9941f5b0c3ccf1b5ce71e24577dc/ros2.repos
BUILD args: --packages-above-and-dependencies ros2topic
TEST args: --packages-above ros2topic
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/14521

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