Add a utility for spawning subprocesses

[mjcarroll]

🎉 New feature

Summary

This includes the unlicensed header-only subprocess.h from https://github.com/sheredom/subprocess.h

This gives a uniform, cross-platform way of spawning executables and interacting with them from a C++ application. Importantly, it allows for reading both cout and cerr.

The intention is to use this to remove any custom executable calling that we have throughout the remainder of the Gazebo stack.

Test it

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.

[codecov]

Codecov Report

Merging #98 (eed4bd2) into gz-utils2 (135a6f6) will decrease coverage by 12.52%.
The diff coverage is 70.61%.

❗ Current head eed4bd2 differs from pull request most recent head 99926b7. Consider uploading reports for the commit 99926b7 to get more accurate results

@@              Coverage Diff               @@
##           gz-utils2      #98       +/-   ##
==============================================
- Coverage      91.97%   79.45%   -12.52%     
==============================================
  Files              6        8        +2     
  Lines            137      331      +194     
==============================================
+ Hits             126      263      +137     
- Misses            11       68       +57     

Impacted Files	Coverage Δ
include/gz/utils/detail/subprocess.h	59.25% <59.25%> (ø)
include/gz/utils/Subprocess.hh	96.61% <96.61%> (ø)

[ahcorde]

Just a small comment