Recognize "ArnoldProceduralCustom" on export for Maya-usd

Hello everyone,

Recently we've been testing viability of using Yeti Fur (https://peregrinelabs.com/products/yeti) for rending hair and fur.

We noticed that the Arnold-USD plugin will recognize the "pgYetiMaya" nodes as an "ArnoldProceduralCustom" on export, but when using the Maya-usd the result is just an empty Xform prim.

I'm currently testing with:

Windows 10
Maya 2024.1
Maya-usd 0.23.0
MtoA 5.3.1.1
Yeti 4.2.7

Example Export from Maya-usd:

```
def Xform "hair_test_master" (
kind = "assembly"
)
{
def Xform "hair"
{
def Xform "body_fur"
{
}
}
}
```

Example Export from Arnold-USD

```
def Xform "hair_test_master"
{
def Xform "hair"
{
def Xform "body_fur"
{
def ArnoldProceduralCustom "body_furShape"
{
float arnold:density = 10
string arnold:filename = "/PATH/TO/CACHE/hair_test.%04d.fur"
int arnold:frame.timeSamples = {
1: 1,
2: 2,
3: 3,
4: 4,
5: 5,
6: 6,
7: 7,
8: 8,
9: 9,
10: 10,
}
uint arnold:id = 1155865197
float arnold:motion_end = 0
string arnold:node_entry = "pgYetiArnold"
float[] arnold:samples.timeSamples = {
1: [1],
2: [2],
3: [3],
4: [4],
5: [5],
6: [6],
7: [7],
8: [8],
9: [9],
10: [10],
}
int arnold:verbosity = 1
rel material:binding =
string primvars:dcc_name = "body_furShape" (
elementSize = 1
interpolation = "constant"
)
float primvars:min_pixel_width = 0 (
elementSize = 1
interpolation = "constant"
)
int primvars:mode = 0 (
elementSize = 1
interpolation = "constant"
)
string[] primvars:mtoa_shading_groups = ["fur_SG"] (
elementSize = 1
interpolation = "constant"
)
}
}
}
}
```

The results from the Arnold-USD export while not renderable within Maya or Houdini Solaris (as far as I can manage anyway), it is renderable via kick.

**Describe the solution you'd like**
Maya-usd export with the Arnold "plugin configuration option" would recognize and export the "ArnoldProceduralCustom" nodes.

**Describe alternatives you've considered**
Currently a reason why we wouldn't just use the Arnold-USD export is in the way the export structures the layer. For example we find materials are added to the root instead of like how the Maya-usd export works where it will create a scope under the default prim. This is more convenient when multiple assets are referenced into a stage.

I'm hoping the Maya-usd plugin will support exporting "ArnoldProceduralCustom" nodes and follow the same pattern it does with materials and how they are binding, merging shapes and transforms etc.

I appreciate any insight into this and apologize if this should not be tagged as a feature request.

Cheers,

Jason Coelho

Start by reproducing the Maya-usd export with the listed Maya, Maya-usd, MtoA, and Yeti versions, then compare its empty Xform output with the Arnold-USD example. Trace the Maya-usd plugin configuration and export handling for ArnoldProceduralCustom nodes. Done means the configured export preserves the procedural node and its relevant attributes while retaining Maya-usd's layer and material structure.

Written by the indexing model from the issue text.