feat(InteractableObject): add a rigid body if missing

If no rigid body is found when InteractableObject starts up, it will
add one, but make sure the rigid body is set to 'kinematic' so it
doesn't start unexpectedly rolling around, popping up, or falling.

Author: bjennings76

Assets/SteamVR_Unity_Toolkit/Scripts/VRTK_InteractableObject.cs

@@ -254,6 +254,13 @@ public void ZeroVelocity()
         protected virtual void Awake()
         {
             rb = this.GetComponent<Rigidbody>();
+
+            // If there is no rigid body, add one and set it to 'kinematic'.
+            if (!rb)
+            {
+                rb = gameObject.AddComponent<Rigidbody>();
+                rb.isKinematic = true;
+            }
         }
 
         protected virtual void Start()