Adds a context pointer to subscriptions (backport #107)

[mergify]

This is an automatic backport of pull request #107 done by Mergify.
Cherry-pick of 5b11c5a has failed:

On branch mergify/bp/galactic/pr-107
Your branch is up to date with 'origin/galactic'.

You are currently cherry-picking commit 5b11c5a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
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Changes to be committed:
	modified:   NOTICE
	modified:   rclc/include/rclc/executor.h
	modified:   rclc/include/rclc/executor_handle.h
	modified:   rclc/src/rclc/executor.c
	modified:   rclc/src/rclc/executor_handle.c
	modified:   rclc/test/rclc/test_executor.cpp
	new file:   rclc_examples/src/example_sub_context.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 #120 (c3b075b) into galactic (88cd65c) will decrease coverage by 0.51%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           galactic     #120      +/-   ##
============================================
- Coverage     32.55%   32.03%   -0.52%     
============================================
  Files            31       32       +1     
  Lines          3600     3786     +186     
  Branches       1791     1850      +59     
============================================
+ Hits           1172     1213      +41     
- Misses          932     1032     +100     
- Partials       1496     1541      +45     

Impacted Files	Coverage Δ
.../src/zmy3q7fyodg/rclc/rclc/test/rclc/test_node.cpp
...fyodg/rclc/rclc/test/rclc/test_executor_handle.cpp
...lc_parameter/src/rclc_parameter/parameter_server.c
...c/zmy3q7fyodg/rclc/rclc/test/rclc/test_service.cpp
ros_ws/src/zmy3q7fyodg/rclc/rclc/src/rclc/init.c
.../zmy3q7fyodg/rclc/rclc/test/rclc/test_executor.cpp
...rclc_lifecycle/src/rclc_lifecycle/rclc_lifecycle.c
ros_ws/src/zmy3q7fyodg/rclc/rclc/src/rclc/client.c
...yodg/rclc/rclc_examples/src/example_service_node.c
ros_ws/src/zmy3q7fyodg/rclc/rclc/src/rclc/sleep.c
... and 53 more

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[JanStaschulat]

solves #50 for galactic