USDShader : Add new node for loading shaders from pxr::SdrRegistry

[johnhaddon]

This provides the ability to load and assign the various UsdPreviewSurface shaders, and will form the foundations for loading UsdLuxLights in a future PR.

We have been debating whether or not we should be using the SdrRegistry or the SchemaRegistry for loading this stuff, and I came down on the side of the SdrRegistry for now because :

• The UsdPreviewSurface definitions are only registered with the SdrRegistry, not the SchemaRegistry.
• After building arnold-usd, I couldn't find any useful "bolt on" schemas that would be useful for extending a UsdLuxLight for Arnold.
• Nothing about the implementation of UsdShader is exposed publicly, so we can switch to loading from the schema registry in future if we want, without any visible change.

[murraystevenson]

Thanks John! A couple of minor comments and questions inline, but otherwise is looking good to me.

[johnhaddon]

Thanks for the review - I've pushed fixups and replied to each of your comments. The initial commits are as they were before, but have changed SHA just because I rebased to get Changes.md in sync with the 1.3 release. It should be sufficient to only re-review the fixups.

[murraystevenson]

Thanks John, fixups look good to me! Should be good to go after ensuring we're targeting 1.3_maintenance rather than main.