[ign to gz] C++ namespaces

[chapulina]

🎉 New feature

• Part of Rename Ignition to Gazebo gazebo-tooling/release-tools#698
• Similar to [ign to gz] C++ namespaces gz-utils#50
• Subtask: C++ namespaces, directories and header guards from ignition to gz gazebo-tooling/release-tools#711

Summary

Changes:

• Change all namespaces from ignition to gz
• Add alias so that ignition still works for users (tick-tock's tick)
• Added a test to make sure ignition still works

Test it

See the added test. I also expect downstream libraries not to need any modifications because of this PR.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial - We'll add complete migration tutorials at the end
• 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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[methylDragon]

Superceded by #83

[chapulina]

See #83