Add plugins namespace

[mabelzhang]

🦟 Bug fix

Add namespace so gz::gui::plugins can be used as a \ref link in gz-sim Doxygen tutorials.

Summary

Per gazebosim/gz-sim#2107 (comment)

Braces same line or breakline - I followed the format here
https://github.com/gazebosim/gz-sim/blob/9d79b629c9e0fed1c8635e30f06b2c61462f32d8/include/gz/sim/System.hh#L40

Checklist

• Signed all commits for DCO
• Added tests
• 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 #571 (6357740) into gz-gui8 (e115d56) will not change coverage.
The diff coverage is n/a.

❗ Current head 6357740 differs from pull request most recent head 91f9750. Consider uploading reports for the commit 91f9750 to get more accurate results

@@           Coverage Diff            @@
##           gz-gui8     #571   +/-   ##
========================================
  Coverage    68.10%   68.10%           
========================================
  Files           38       38           
  Lines         5367     5367           
========================================
  Hits          3655     3655           
  Misses        1712     1712           

Files Changed	Coverage Δ
include/gz/gui/Plugin.hh	100.00% <ø> (ø)

[mabelzhang]

ABI check is failing 🤔 But I'm not seeing anything obvious on Jenkins. There isn't an ABI report...?

[azeey]

ABI check is failing 🤔 But I'm not seeing anything obvious on Jenkins. There isn't an ABI report...?

ABI checker is not working on Harmonic yet. It can be ignored for now.