[ign to gz] Factory::New accept ignition with warning

[chapulina]

• Part of C++ namespaces, directories and header guards from ignition to gz gazebo-tooling/release-tools#711
• Closes gz-transport12 ignition.msgs backward compatibility gz-transport#331

Summary

Accept ignition messages on Factory::New, print warning and return gz equivalent.

Test it

Try sending an ignition message, see that it's sent, and a warning is printed.

ign topic -t /model/joint/thruster4_joint/cmd_thrust -m ignition.msgs.Double -p 'data: 5'
Trying to create deprecated message type [ignition.msgs.Double]. Use [gz.msgs.Double] instead.

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.

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[methylDragon]

Provisionally approving if the comment I left earlier can be disregarded :o

[codecov]

Codecov Report

Merging #281 (831525e) into main (0a09d3d) will decrease coverage by 0.97%.
The diff coverage is 62.85%.

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
- Coverage   85.59%   84.62%   -0.98%     
==========================================
  Files          10       10              
  Lines         972      995      +23     
==========================================
+ Hits          832      842      +10     
- Misses        140      153      +13     

Impacted Files	Coverage Δ
include/gz/msgs/Factory.hh	66.66% <60.00%> (-33.34%)	⬇️
src/Factory.cc	81.18% <64.00%> (-7.19%)	⬇️

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