Softness does not support per limit settings, Force Based, or Stiffness and Damping

See Godot-Rapier-Physics issue:
[Rapier Lib softness does not support per limit settings, Force Based, or Stiffness and Damping #546](https://github.com/appsinacup/godot-rapier-physics/issues/546)

Rapier softness settings cannot support Godot parameters (even if Godot upgrades their parameters in future)

1. The Rapier softness setting applies across all locked axes and limits, rather than per limit
2. The setting only offers frequency and damping ratio parameterisation (not stiffness, damping)
3. The setting doesn't offer a Force Based mode (presumably it is Acceleration Based)

All the above would need to be addressed to support this proposal:

[Unify Joint3D spring parameters and make them understandable #14845](https://github.com/godotengine/godot-proposals/issues/14845)

Or in stepping towards support for the following glTF extension proposal:

[Pull Request for KHR_physics_rigid_bodies](https://github.com/eoineoineoin/glTF/blob/rigidBodyMerge/extensions/2.0/Khronos/KHR_physics_rigid_bodies/README.md#joints)

I don't think there is much of a case to reshape the Godot proposal because:
* Godot already has these softness per limit (though no one can replicate them with modern parameters)
* They may be useful for rag dolls in 3D