Would be useful to have a 3D spring joint

[blackears]

At the moment, Godot has a spring joint for 2D but not 3D. I've seen some discussion that you can get spring behavior using a Generic6DOFJoint, but this joint is not well documented and I can't find any good examples of how to set it up. Even if it is possible with a Generic6DOFJoint, I would feel that spring behavior is common enough that a dedicated spring joint would be useful all on its own.