IRigidbodyRef Interface
This interface characterizes a type for which the most relevant Unity Rigidbody can be on an arbitrary GameObject, not necessarily on the same GameObject as the current instance.
For example, a HandGrab.HandGrabInteractor component may be placed on a separate GameObject from the Rigidbody it uses for its internal logic. This interface provides a unified way of accessing the relevant Rigidbody.
The Unity Rigidbody most relevant to the current instance.
The role this Rigidbody plays varies according to the implementing type; for example, an HandGrab.HandGrabInteractor may use a Rigidbody for collision detection when grabbing.
