Example real-time concurreny timer and subscription (backport #329)

[mergify]

This is an automatic backport of pull request #329 done by Mergify.
Cherry-pick of fd648d9 has failed:

On branch mergify/bp/foxy/pr-329
Your branch is up to date with 'origin/foxy'.

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  (fix conflicts and run "git cherry-pick --continue")
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Changes to be committed:
	modified:   rclc_examples/README.md
	new file:   rclc_examples/src/example_short_timer_long_subscription.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   rclc_examples/CMakeLists.txt

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[codecov-commenter]

Codecov Report

Merging #331 (f8860e2) into foxy (02472d6) will decrease coverage by 2.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             foxy     #331      +/-   ##
==========================================
- Coverage   62.93%   60.32%   -2.62%     
==========================================
  Files          13       14       +1     
  Lines        1419     1555     +136     
  Branches      416      507      +91     
==========================================
+ Hits          893      938      +45     
- Misses        337      339       +2     
- Partials      189      278      +89     

Impacted Files	Coverage Δ
...c_parameter/test/rclc_parameter/test_parameter.cpp	33.08% <0.00%> (ø)

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