Add proto message for MaterialColor.

[bperseghetti]

🎉 New feature

Summary

The MateriaColor proto message allows for changing the material colors of an entities visual component easily and can even be done from ROS using the gz bridge.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [x] Added example and/or tutorial
• [x] Updated documentation (as needed)
• [x] Updated migration guide (as needed)
• [x] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (47eed13) 97.67% compared to head (5c03d8a) 97.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-msgs10 #414 +/- ## ========================================== Coverage 97.67% 97.67% ========================================== Files 27 27 Lines 1163 1163 ========================================== Hits 1136 1136 Misses 27 27 ```

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