[DRAFT] Add PICO Body tracking support

[BastiaanOlij]

This PR implements XR_BD_body_tracking extension, PICO's body tracking extension that is available for users who have purchased PICOs motion trackers.

It works similar to Metas body tracking and provides a completely posed skeleton for our avatar. The main difference is that we do not get hand tracking combined.

This is mostly working, the main issue is that a number of joints do not align with our humanoid character. With bone update mode set to "Rotation Only" the result is usable, but with positioning taken into account our lovely Kun deforms at the legs and around the neck/shoulder area:

Todos before this is merge-able:

• Come up joint offset logic that will resolve the deformations.
• Figure out why the hand joints aren't being animated (might be an issue on PICO side).
• See if we can combine this with XRHandTracking to get posed hands and add that to the demo.
• Combine the META and PICO body tracking demos into one demo that adds a bit of boiler plate logic to enable the right settings depending on what the tracker reports as supported.