mayaUsd.lib.SchemaApiAdaptor is not triggered when importing a Material (and related) prims

**Describe the bug**

When importing a USD file containing a `Mesh` with a `Material`, any registered `SchemaApiAdaptor` is not triggered for the `Material` and related concepts.

**Steps to reproduce**

Steps to reproduce the behavior:

1. Load the `metadata_plugin.py` by unzipping `plugin.zip` and adding it to the `MAYA_MODULE_PATH`
2. Import `textured_cube.usda` from the `asset.zip`
3. Note that the adaptor logic was only called for the Mesh and not for the material or texture:


**Expected behavior**

My registered `SchemaApiAdaptor` is triggered for the `Material` and other child prims, so it would print the following as well:

```
cuba_initialShadingGroup: CanAdaptForImport
cuba_initialShadingGrouppCube: ApplySchemaForImport
cuba_initialShadingGroup | cuba_initialShadingGrouppCube1: CopyFromPrim
```

Or some variation of it.

**Attachments**

* [example_plugin.zip](https://github.com/Autodesk/maya-usd/files/11445243/example_plugin.zip): Code to show the encountered bug
* [asset.zip](https://github.com/Autodesk/maya-usd/files/11444877/asset.zip): simple cube used to illustrate the issue

**Specs (if applicable):**
- OS & version: Windows 10
- Maya version: Maya 2020, 2022, 2023
- Maya USD commit SHA: dev at b2e4736c83ce5e3f8deebc4fcf0729a8174f3d3d

**Additional context**

The context for wanting to run `SchemaApiAdaptor` instances for our `Material` prims is that we have a custom API schema that indicates the prim contains specific `AssetInfo`, we copy this data to Maya in a structured format that allows the artists to inspect it, and if necessary update it. Because the `SchemaApiAdaptor` is not triggered, the metadata is not correctly copied, which leads to issues down the line of our pipeline/workflow.

From my understanding, the `Material` is interpreted as a `shadingEngine` node, which inherits from the `entity` type so it should work, however when debugging Maya-USD, I saw that the shader network is actually read as part of the `Mesh` reader, and then later on visited again. I am not sure if this has an impact on the current behavior.

The current workaround I see is to implement logic in a chaser that explicitly retrieves any `Material` by following the links in meshes/shapes, and then finding the corresponding created Maya nodes. This leads to a lot of low-level logic that I believe should instead be provided by Maya-USD, so ideally I would not need to resort to such an approach.