Robustify spawner (backport #1501)

[mergify]

With the current implementation it can happen that when using multiple controller spawners some of them fail or get stuck, see #1182. This fixes #1182 and #1483.

While writing this, I realize this has great overlap with #1483, but I see no problem combining those two.

As briefly mentioned above, this change addresses two things:

• The node discovery mechanism for the controller manager seems to be error-prone and not strictly needed.
• It can happen that we don't get a response from the service server and hang in a deadlock.

Note: The test I implemented is a bit hacky, so we might also want to remove it again? Otherwise I think we would have to change the following things:

• I installed the test folder in order to access the controller configuration file living in there. We should either install that file by hand rather than the complete test folder or move it somewhere else.
• I've added a urdf file in ros_control_test_assets as I thought that might be the most useful place. I might be wrong.
• I've added a sleep to my test before checking whether the controller_manager shows up all the expected controllers. That worked for me for an implementation, but since a timeout is very error-prone it would be better to have a proper waiting mechanism. With the changes I made the spawners should die eventually and not hang in a deadlock, so we could add event handlers to the launch description, but I'm not sure how to combine the exit events from all the spawners into one trigger.

I implemented and tested things on the rolling on jammy installation I currently have, but I know the problem definitively also arises for humble users.

---

This is an automatic backport of pull request #1501 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   controller_manager/controller_manager/controller_manager_services.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller_manager/test/test_spawner_unspawner.cpp

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 62.09%. Comparing base (41723d2) to head (aae3fb1).
Report is 1 commits behind head on iron.

Files	Patch %	Lines
controller_manager/test/test_spawner_unspawner.cpp	75.00%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1687      +/-   ##
==========================================
+ Coverage   62.08%   62.09%   +0.01%     
==========================================
  Files         102      102              
  Lines       11775    11791      +16     
  Branches     8447     8461      +14     
==========================================
+ Hits         7310     7322      +12     
+ Misses        747      745       -2     
- Partials     3718     3724       +6     

Flag	Coverage Δ
unittests	62.09% <75.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
controller_manager/test/test_spawner_unspawner.cpp	57.32% <75.00%> (+0.94%)	⬆️

... and 1 file with indirect coverage changes