Expose the API for extracting data from PLY files

[vilya]

We already have support for loading a PLY file and returning a TriangleMesh primitive, but this is very rigid: it relies on the file having a specific set of sections each with a specific set of properties and doesn't allow custom properties, or custom data from additional sections, to be imported.

Internally minipbrt uses miniply to load the PLY files, which has all these capabilities. We should simply expose that as part of minipbrt's API.