Adds a context pointer to subscriptions (backport #107)

[mergify]

This is an automatic backport of pull request #107 done by Mergify.
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On branch mergify/bp/master/pr-107
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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 #121 (5b11c5a) into master (8697d7d) will decrease coverage by 0.88%.
The diff coverage is n/a.

❗ Current head 5b11c5a differs from pull request most recent head 4ee3509. Consider uploading reports for the commit 4ee3509 to get more accurate results

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   29.56%   28.68%   -0.89%     
==========================================
  Files         243       28     -215     
  Lines       24444     3246   -21198     
  Branches    12312     1679   -10633     
==========================================
- Hits         7227      931    -6296     
+ Misses       6885      914    -5971     
+ Partials    10332     1401    -8931     

Impacted Files	Coverage Δ
...aw/rclc/rclc_examples/src/example_lifecycle_node.c
...c/71b65q9ngn7/rclc/rclc/src/rclc/executor_handle.c
ros_ws/src/71b65q9ngn7/rclc/rclc/src/rclc/init.c
...rclc_lifecycle/src/rclc_lifecycle/rclc_lifecycle.c
ros_ws/src/71b65q9ngn7/rclc/rclc/src/rclc/timer.c
...rclc_lifecycle/src/rclc_lifecycle/rclc_lifecycle.c
...5apnlmh7/rclc/rclc_examples/src/example_executor.c
.../src/709f64qbexs/rclc/rclc/test/rclc/test_node.cpp
ros_ws/src/vp5apnlmh7/rclc/rclc/src/rclc/timer.c
...os_ws/src/7aqjevewu6/rclc/rclc/src/rclc/executor.c
... and 261 more

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[JanStaschulat]

solves #50 forRolling