Use common::findFile when searching for slip map in LookupWheelSlip system

[iche033]

🦟 Bug fix

Summary

The lookup wheel slip system currently only searches the slip map file by checking path relative to the world file. This PR updates the file search logic to use common::findFile which is more reliable. It now searches for paths relative to the model sdf file (if plugin is added to a model in model.sdf) and world file, as well as URIs pointing to Fuel.

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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