Drop requirement on `utils-urdf` (CMake DART component)

[roncapat]

The URDF loader is only used in a test, not the gz_physics core.
Would it be possible to find_package() such component only when building tests?

[azeey]

This makes sense to me. I'll add it the help wanted label. If you're interested in contributing a PR let me know.

[roncapat]

Thanks! I may try in some spare time, but if somebody else reads this and wants to pick it up, if you see no branch/PR linked from myself please go on :)