Backport Node::CreateSubscriber API (backport #622)

[mergify]

🎉 New feature

Summary

Backport the new Node::CreateSubscriber API function added in #608.

This PR does not backport the refactoring of the Node::Subscribe functions as that introduces a break change as mentioned in gazebosim/gz-sim#2894

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

---

This is an automatic backport of pull request #622 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of dea4643 has failed:

On branch mergify/bp/gz-transport13/pr-622
Your branch is up to date with 'origin/gz-transport13'.

You are currently cherry-picking commit dea4643.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Migration.md
	modified:   include/gz/transport/Node.hh
	modified:   include/gz/transport/detail/Node.hh
	modified:   src/Node.cc
	modified:   src/NodePrivate.hh
	modified:   src/NodeSharedPrivate.hh
	modified:   src/Node_TEST.cc
	modified:   test/CMakeLists.txt
	modified:   test/integration/CMakeLists.txt
	new file:   test/integration/test_executables/twoProcsPubSubMixedSubscribers_aux.cc
	modified:   test/integration/twoProcsPubSub.cc
	modified:   test/test_config.hh.in

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/gz/transport/NodeShared.hh
	both modified:   src/NodeShared.cc

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

[codecov]

Codecov Report

Attention: Patch coverage is 86.82635% with 22 lines in your changes missing coverage. Please review.

Project coverage is 86.73%. Comparing base (eac2e69) to head (a1a210a).
Report is 40 commits behind head on gz-transport13.

Files with missing lines	Patch %	Lines
src/NodeShared.cc	78.72%	20 Missing ⚠️
include/gz/transport/detail/Node.hh	94.11%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           gz-transport13     #623      +/-   ##
==================================================
- Coverage           87.69%   86.73%   -0.96%     
==================================================
  Files                  59       59              
  Lines                5704     5902     +198     
==================================================
+ Hits                 5002     5119     +117     
- Misses                702      783      +81     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.