XRHandJoint:
| "wrist"
| "thumb-metacarpal"
| "thumb-phalanx-proximal"
| "thumb-phalanx-distal"
| "thumb-tip"
| "index-finger-metacarpal"
| "index-finger-phalanx-proximal"
| "index-finger-phalanx-intermediate"
| "index-finger-phalanx-distal"
| "index-finger-tip"
| "middle-finger-metacarpal"
| "middle-finger-phalanx-proximal"
| "middle-finger-phalanx-intermediate"
| "middle-finger-phalanx-distal"
| "middle-finger-tip"
| "ring-finger-metacarpal"
| "ring-finger-phalanx-proximal"
| "ring-finger-phalanx-intermediate"
| "ring-finger-phalanx-distal"
| "ring-finger-tip"
| "pinky-finger-metacarpal"
| "pinky-finger-phalanx-proximal"
| "pinky-finger-phalanx-intermediate"
| "pinky-finger-phalanx-distal"
| "pinky-finger-tip"
The XRHand interface is pair iterator (an ordered map) with the key being the hand
joints (XRHandJoint) and the value being an XRJointSpace.