Mujoco (implicit) integration does not offer a mechanism to take into account custom passive forces in `qDeriv` calculation

### The feature, motivation and pitch

While constant stiffness and damping are the most common model for passive forces, there does exist needs for more complex passive forces (piecewise linear, nonlinear, lookup, etc.). Mujoco offers two mechanisms, `mjcb_passive` callback and passive force plugin, to calculate custom passive forces at maximum flexibility. However, in integration step, if I understand correctly, only simple damping is considered in `qDeriv` calculation via `mjd_smooth_vel -> mjd_passive_vel`. This makes the implicit integration less accurate, and may lead to divergence.

### Alternatives

_No response_

### Additional context

_No response_