Remove *ContactsState classes

[diegoferigo]

In the past few months, JaxSim has received numerous updates and additions to contact management. We now have three contact models (soon four), and the only one that requires to extend the ODE state vector that is integrated with the floating-base dynamics is the original SoftContacts model.

• This PR generalizes how additional variables can be introduced to the integrated state vector.
• The material deformation $\mathbf{m}$ used by the SoftContacts model is now treated as an augmented ODE state.
• This generalization allow users to use our integrators with their own custom dynamics (potentially including additional effects not currently considered in JaxSim, e.g. actuator dynamics and muskolo-skeletal models).

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📚 Documentation preview 📚: https://jaxsim--256.org.readthedocs.build//256/

[flferretti]

Thank you Diego! This is great. I just left a single minor comment

[xela-95]

Thanks for the improvement @diegoferigo !! I just left a minor comment