Add the possiblity to set joint_natural_frequency and joint_damping_ratio per joint

[Ughuuu]

Right now one can set these 2 parameters per world, but not per joint. Having this setting available per joint would make the setting more versatile, making it possible to make other joints have soft limits, not just spring joints.

[Ughuuu]

This would help with closing gap the godot pin joint feature (godot pin joint has softness parameter from 0 to 1). Rapier is currently missing this.