Support fingertracking

[Erimelowo]

Support fingertracking in the skeleton.
Then, send it via VMC and OpenVR (#1085) (OpenVR support may be its own PR)

There are 30 bones in total (15 per hand; 3 per finger). We should support anywhere from 1 to 3 tracker/data point per finger (if lower than 3, just do some more-or-less fancy math to figure out the rotation of the other finger bones)

distal = tip of finger
proximal = first bone of finger

Fingers also need lengths for BVH recordings, tho we may get away with calculating it from the user's arm length or similar since it shouldn't actually matter afaik

[Eirenliel]

I think we should add a bit more so the server also knows that the whole device is a glove

[ImUrX]

i told that to Erimel, our conversation in summary was, maybe we want to support an external's system finger tracking bones, but this shouldnt really be assigned by the end user 😭