Add support for a spring and a joint on the same axis for godot impl.

[Stefan-5422]

appsinacup/godot-rapier-physics#459

Godot exposes a Generic6DOFJoint3D which has both a motor and a spring on each of the 6 DOFs which, does not seem to be possible with the current 1 motor per DOF which is also used for a spring model.

Expected behaviour (simulated with Jolt3D):

untitled.mp4