Refactor `integrators` module and redefine `JaxSimModel` class attributes

[flferretti]

This pull request includes several changes to the integrators logic and API. The JaxSimModel class now incorporates integrator and dt as static attributes. In order to do this, it was necessary to refactor the jaxsim.integrators module in order to simplify the logic by removing the init method.

The integrators logic is simplified by avoiding the use of integrator_state to pass around variables and making wrap_system_dynamics_for_integration accept only the function describing the dynamics of the system, since model and data were required arguments for the integration anyway.

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📚 Documentation preview 📚: https://jaxsim--252.org.readthedocs.build//252/