Add code coverage ignore file

[AzulRadio]

New feature

Part of gz-sim#1575

Summary

Add coverage.ignore.in to ignore vendor code in code coverage report. See gz-cmake#279 for detail.

Increase the coverage rate from ~30% -> ~56%

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 #179 (f00740c) into ign-launch2 (0f6edef) will increase coverage by 26.50%.
The diff coverage is n/a.

❗ Current head f00740c differs from pull request most recent head 6fc93b1. Consider uploading reports for the commit 6fc93b1 to get more accurate results

@@               Coverage Diff                @@
##           ign-launch2     #179       +/-   ##
================================================
+ Coverage        30.24%   56.74%   +26.50%     
================================================
  Files                3        2        -1     
  Lines              734      326      -408     
================================================
- Hits               222      185       -37     
+ Misses             512      141      -371     

Impacted Files	Coverage Δ
src/vendor/backward.hpp

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