Add color support for robot meshes in Viser

[AlexyLegrand]

Thanks for taking the time to make and fill out this pull request!

Description

This PR adds support for loading and displaying mesh colors in Viser, improving the visual fidelity of robot models.

Checklist

• I have run pre-commit run --all-files or pixi run lint
• [ ✓] I have performed a self-review of my own code
• [ ✓] I have commented my code where necessary
• I have made corresponding changes to the doxygen documentation
• I have added tests that prove my fix or feature works
• I have updated the CHANGELOG or added the "no changelog" label if it's a CI-related issue
• I have updated the README credits section

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
• build_casadi (build Pinocchio with CasADi support)
• build_autodiff (build Pinocchio with CppAD support)
• build_codegen (build Pinocchio with CppADCodeGen support)
• build_extra (build Pinocchio with extra algorithms)
• build_mpfr (build Pinocchio with Boost.Multiprecision support)
• build_sdf (build Pinocchio with SDF parser)
• build_accelerate (build Pinocchio with APPLE Accelerate framework support)
• build_all (build Pinocchio with ALL the options stated above)

Thanks.
The Pinocchio development team.

[sea-bass]

Thanks for helping improve the minimal viser functionality I added -- some minor comments.

[AlexyLegrand]

Hi, any update on this?

[jorisv]

Hello @AlexyLegrand,
I will review your PR by the end of the week.

[AlexyLegrand]

great, thank you !