feat: USD Codec

[jasonjgardner]

I've ported the USDZ export feature from my PBR plugin into its own codec plugin. Pretty straightforward.

From Blockbench:

To Adobe Substance 3D Stager:

[JannisX11]

I was having trouble exporting models without PBR material, the elements with default textures were missing.
Also there seems to be no support for bones / hierarchy.
This this is now a standalone plugin, those two caveats should either be resolved, or mentioned in the plugin about page, since people will expect this to work as a general purpose 3D export format.
Additionally, the "await_loading": true option can be removed from the plugin config, since the plugin does not need to be active during startup.
Otherwise looks good!