Use `std::shared_ptr` for `gz::transport::NodeShared`

[azeey]

🦟 Bug fix

Summary

Currently, NodeShared is never destroyed once it's created. This makes it hard for writing tests that do not interfere with each other. This patch uses a reference counted smart pointer (std::shared_ptr) to keep track of NodeShared instances, so that it gets properly destroyed when when all Node instances, which themselves contain NodeShared instances are destroyed.

This was brought up in https://github.com/gazebosim/gz-sim/pull/2334. The INTEGRATION_triggered_publisher test fails reliably in my local tests without this gz-transport PR.

~TODO: This is probably not threadsafe. It's possible to lock a mutex before checking the weak_ptr, but that would make the call to NodeSHared::Instance slower than the previous implementation since it was using std::shared_mutex.~

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] 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.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 87.85%. Comparing base (eac2e69) to head (1f2d6f4). Report is 6 commits behind head on gz-transport13.

Files	Patch %	Lines
src/NodeShared.cc	91.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-transport13 #484 +/- ## ================================================== + Coverage 87.69% 87.85% +0.16% ================================================== Files 59 59 Lines 5704 5708 +4 ================================================== + Hits 5002 5015 +13 + Misses 702 693 -9 ```

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[azeey]

I can go ahead and merge this, but I think we should make a prerelease and test gz-sim before making a full release. @iche033 or @caguero can I ask one of you to do that please?