Add USD mesh loader to MaterialX tools

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.

Start by locating the MaterialXGraphEditor and MaterialXViewer loading entry points and reviewing how optional build dependencies are configured. Define the initial scope as loading meshes only, with OpenUSD optional at build time, and verify both tools can open a USD mesh without making OpenUSD mandatory for other users.

Written by the indexing model from the issue text.