Add new physics elements for Nvidia PhysX

## Desired behavior

I hope SDFormat would support the PhysX(Nvidia) as a physic engine so that I can utilize the standard format for robot simulation.

Since Nvidia PhysX introduced a new articulation body for robotics, and I think it's enough and possible to present the robot s in the world what we imagine.

refer to example [here](https://github.com/Unity-Technologies/articulations-robot-demo).

[TGS](https://gameworksdocs.nvidia.com/PhysX/4.0/documentation/PhysXGuide/Manual/RigidBodyDynamics.html#temporal-gauss-seidel) is provided for solver in PhysX.

I guess many developers already know these feature in PhysX 4.0 but current SDFormat(even latest version) does not support that type in official. :(

Anyone have the plan to introduce a new physics engine type?

## Implementation suggestion

describe any parameters related to PhysX

## Additional context

Actually I'm working on open project which called [CLOiSim](https://github.com/lge-ros2/cloisim) to simulate the robotics simulation using SDFormat.
I couldn't find reasonable SDFormat parsing library for C#, so I've created the parser by my self.

And it implements the physics things like joint, inertial and whatever.. using the parameters in ODE type.