Add USD mesh loader to MaterialX tools

[ld-kerley]

It would be super handy to be able to load in USD Meshes in to MaterialXGraphEditor and MaterialXViewer.

I am imagining this to be an optional build time dependency, as we don't want to require all users of MaterialX to provide OpenUSD.

The OpenUSD file format is complex, and I would propose restricting this to just loading a meshes initially, this could potentially be expanded in the future.

Adding support for loading OpenUSD meshes, would potentially provide a route to loadin ga wide number of file formats, if those are loadable by the OpenUSD.

[jstone-lucasfilm]

This is a great suggestion, @ld-kerley, and for completeness I'll note that there are two potential paths forward:

1. Adding geometric USD support through an optional dependency on OpenUSD.
2. Adding geometric USD support through an optional dependency on TinyUSDZ.

The author of TinyUSDZ has already begun an implementation of option (2) in a fork of MaterialX, and we can try out this option here:

https://github.com/lighttransport/materialx