Wrist attachment and standard controller mount points

No rush, but I think a VRTK_WristAttachment or a left and right wrist alias object in the SDK Manager could be a useful feature. Watches are a useful interface that we may see more of in VR; they can tell in game time / level timers / or work as a menu interface. A few games have done this so far just with a watch on a hand model. But it would be good to have a point we can simply attach something to to have it follow the spot your wrist should be in.

In general it would probably be nice to have a standard set of attachment transform points available for controllers (eg: a grab transform for the relative location your hands grab around, to lock a pistol grip or sword handle to; a barrel transform for the location a gun barrel would naturally point and you'd expect a generic gun to shoot towards; a point transform for the direction a laser pointer would feel natural with the controller, for Vive wands that's usually upwards from the wand while on Touch you normally point with an index finger) since they vary by controller.

However I think it's worth special casing the wrist transform and giving it a dedicated script. It's more of a fixed attachment point rather than a point relative to grabbing. Also having a script dedicated to the wrist leaves us room to support any changes to how the wrist location is tracked. We've already seen tracked gloves that will be coming out. So it's possible that in the future the wrist and controller could be part of separate tracked objects. Also the gloves shown off seem to track how the wrist bends, so the location of the wrist relative to the controller is variable.