Bring back schema library as an optional component to build

**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.

Start by reviewing pull request 706 and the current build configuration for the components whose dependencies are already located. Check how usdGenSchema generates the schema library source and define the optional build component so the compiled library and its Python bindings are produced without affecting other components.

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