Godot 6DOF joint needs a swing twist joint with pyramidal constraint

[appsinacup/godot-rapier-physics#543](https://github.com/appsinacup/godot-rapier-physics/issues/543)

When working on the 6DOF joint, I was unable to find suitable functionality in Rapier lib to implement the same swing twist joint that Jolt does. The swing twist joint with a pyramid cone constraint is necessary in order to implement limits that make sense to the user when they have more than one angular degree of freedom at a time.

There may be other joint limit and motor compositions that can work there. I don't know. I do know that independent axes in the Rapier Generic Joint will do strange things from the user perspective.

I believe one way to fix it would be if Rapier had some sort of coupled axes feature but with two angles and a pyramid constraint (instead of just the cone constraint).