Primvar Support in LookdevX+VP2

**Describe the bug**
We're seeing several issues with the use of PrimVars when using LookDevX in Maya 2024.2 with Viewport 2.0 on a Mac. I assume these limitations extend to other platforms too.

I believe this is a Maya USD side issue rather than LookDevX, but it manifests itself through use of LookDevX hence the title.

The attached file has an example usda and usdz file.

## UV Map Selection

The UV Map selection via `UsdPrimvarReader` does not display correctly when more than one reader is in play.
Here we have a quad with two UV sets: `st` and `st1`. `st` maps textures from 0-1 in UV space, while `st1` maps them in a diagonal. Please refer to the USDZ in the file

`st` or `map1`

`st1` or `map2`

Now, if you consider this network with the following nodes: UsdPrimvarReader, UsdUVTexture, UsdPreviewSurfaceShader

In the image above, a primvar reader for `st1` outputs this varname into the emissive and the base color, while a reader for `st` is connected into occlussion.

In viewport, however, base color and emissive appear to be using `st`, while occlusion uses `st1`, but the opposite should be the case.


The same asset works properly in Preview on macOS


Trying the same with MaterialX nodes almost works, but occlussion doesn't seem to work with the following nodes: ND_geompropvalue_vector2, ND_image_color3, ND_UsdPreviewSurface_surfaceshader)


## Custom Primvars

In another example, we create a `uniform color3`f primvar in a bound mesh, and refer to it also via a `Usd Primvar Reader`, or a `geompropvalue` using MaterialX.

See the attached usda file as an example.

This doesn't seem to work in VP2.0


Switching this to a MaterialX version with `geompropvalue` and `standard surface` doesn't work either


However this works in other MaterialX supported contexts like Houdini


**Steps to reproduce**
Steps to reproduce the behavior:
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4. See error

**Expected behavior**
A clear and concise description of what you expected to happen.

**Attachments**
[Archive.zip](https://github.com/Autodesk/maya-usd/files/14059075/Archive.zip)

**Specs (if applicable):**
- macOS Ventura 13.6
- Maya 2024.2
- Maya USD 0.25.0

**Additional context**
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