Add example of using cmake extras

[mjcarroll]

Demo of using cmake extras

[codecov]

Codecov Report

Merging #97 (2775880) into gz-utils2 (135a6f6) will not change coverage.
The diff coverage is n/a.

❗ Current head 2775880 differs from pull request most recent head 8913acc. Consider uploading reports for the commit 8913acc to get more accurate results

@@            Coverage Diff             @@
##           gz-utils2      #97   +/-   ##
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  Coverage      91.97%   91.97%           
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  Files              6        6           
  Lines            137      137           
==========================================
  Hits             126      126           
  Misses            11       11           

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[mjcarroll]

This is being used by messages now as of harmonic, this was just a proof of concept, closing.