Support <gz-gui> tag in plugin config

[azeey]

🎉 New feature

Summary

This allows users to specify <gz-gui> instead of <ignition-gui> in their GUI plugins so that SDF files can work on Fortress and Garden/Harmonic without modification.

Test it

Run PluginTest.GzSimCompatibility test

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.

[azeey]

Looks like the Focal failures are flaky tests. Windows failed due to infra issues. I'll retrigger it.