Add PointCloudPacked iterators

[chapulina]

🎉 New feature

• Part of Generic point cloud visualization gz-sim#1156

Summary

Our PointCloudPacked message is equivalent to ROS's PointCloud2, but we don't have a good way of handling our message purely from Ignition.

This PR adapts PointCloud2Iterator for Ignition.

Test it

Run the tests

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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #210 (b049cdb) into ign-msgs5 (9c71e5b) will increase coverage by 0.88%.
The diff coverage is 92.70%.

❗ Current head b049cdb differs from pull request most recent head 8446915. Consider uploading reports for the commit 8446915 to get more accurate results

@@              Coverage Diff              @@
##           ign-msgs5     #210      +/-   ##
=============================================
+ Coverage      84.38%   85.27%   +0.88%     
=============================================
  Files              7        9       +2     
  Lines            807      903      +96     
=============================================
+ Hits             681      770      +89     
- Misses           126      133       +7     

Impacted Files	Coverage Δ
include/ignition/msgs/PointCloudPackedUtils.hh	84.21% <84.21%> (ø)
...lude/ignition/msgs/detail/PointCloudPackedUtils.hh	94.80% <94.80%> (ø)

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[caguero]

Looks good to me, just two very minor comments.

[chapulina]

Thanks for the review, @caguero , feedback addressed.

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1