[feature] UsdSkel and blendshapes

[syoyo]

Skinning and blenshapes are important feature for avatars.

We need to support UsdSkel for it: https://graphics.pixar.com/usd/release/api/usd_skel_page_front.html

[syoyo]

Added skinning note to https://github.com/syoyo/tinyusdz/blob/dev/doc/skinning.md

[syoyo]

TinyUSDZ can now construct usdSkel prim as-is: https://github.com/syoyo/tinyusdz/blob/dev/src/usdSkel.hh

Next step is to implement some helper codes: e.g. Skinning Matrix evaluation, Joint hierarchy reconstruction(from linearlized joint array), etc.

[syoyo]

https://graphics.pixar.com/usd/dev/api/_usd_skel__a_p_i__intro.html

Some helper APIs/functions useful to have.

• [ ] GetInheritedSkeleton
• [ ] T.B.W.