Backport gazebo::Util::validTopic() from ign-gazebo4.

[peci1]

🎉 New feature

Supports #869

Summary

Function gazebo::Util::validTopic() has been added in #522 to ign-gazebo4. The function doesn't break any API or ABI, so I think it makes sense to backport it to 3. I made use of this function in #869 while targeted to 4, and now I retargeted to 3 and this function was missing.

I think porting the rest of #522 might change some behavior, so I don't recommend porting the whole PR.

Test it

The unit test has been backported, too.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[codecov]

Codecov Report

Merging #1153 (511a40d) into ign-gazebo3 (6322451) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1153      +/-   ##
===============================================
+ Coverage        77.45%   77.47%   +0.01%     
===============================================
  Files              222      222              
  Lines            12758    12769      +11     
===============================================
+ Hits              9882     9893      +11     
  Misses            2876     2876              

Impacted Files	Coverage Δ
include/ignition/gazebo/Util.hh	100.00% <ø> (ø)
src/Util.cc	93.39% <100.00%> (+0.36%)	⬆️

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[chapulina]

Thanks!