Render models without plugins

The unified XR input API looks promising and it allows us to target any VR platform much more easily. One thing that I'm still missing currently is a way to display the controller models without relying on the SteamVR or Oculus plugins. These plugins provide 3D models of the currently used VR controllers that are updated according to the current input (e.g. trigger and buttons being pushed). They also provide an estimated or accurate hand pose that can be applied to a hand mesh model. This seems to be the last major missing feature from the Unity XR abstraction layer.