Bring back schema library as an optional component to build

[nrusch]

Describe the feature you'd like

This project used to provide an actual schema library (with Python bindings) as a build output, but this was removed in https://github.com/Autodesk/arnold-usd/pull/706.

It would be great if the compiled schema library were brought back as an optional build component, since usdGenSchema is still generating the source code for it, and all the necessary dependencies are already being located to build other components.

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13308