Return only unique message types in MessageFactory::Types function

[iche033]

🦟 Bug fix

Fixes #470

Summary

The MessageFactory::Type function returns duplicate message types. This is because the list of known message types overlaps with types loaded from descriptor files so the function essentially return two of the same message types. This PR uses a set to filter out duplicates. A potential todo is to return an set instead of a vector.

I also removed an extra duplicate check in the New function (probably leftover from ign -> gz migration)

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)
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[VincidaB]

Still getting ERROR Error: duplicate name 'Distortion' in Namespace .gz.msgs on the web visualization.

[iche033]

Still getting ERROR Error: duplicate name 'Distortion' in Namespace .gz.msgs on the web visualization.

addressed on the gz-launch websocket server side: gazebosim/gz-launch#281