mayaUsdProxyShape.filePath missing when querying external file dependencies

**Describe the bug**
The mayaUsdProxyShape has an attribute to refer to a file on-disk however that path is not attribute / attribute value is not discoverable via the Maya API, that I can see right now.

**Steps to reproduce**
Run this script

```python
import tempfile

from maya import cmds
from pxr import Sdf

def _get_all_external_files():
output = set()

directories = cmds.filePathEditor(query=True, listDirectories="", unresolved=True) or []

for directory in directories:
output.update(
cmds.filePathEditor(
query=True,
listFiles=directory,
withAttribute=False,
unresolved=True,
) or []
)

return output

_, path = tempfile.mkstemp(suffix="_some_path.usda")
layer = Sdf.Layer.CreateNew(path)
Sdf.CreatePrimInLayer(layer, "/root")
layer.Save()
node = cmds.createNode("mayaUsdProxyShape")
cmds.setAttr("{node}.filePath".format(node=node), layer.realPath, type="string")
print("external files", layer.realPath in _get_all_external_files())
print("file path attributes", cmds.listAttr(node, usedAsFilename=True))
print("known, used files in the maya scene", layer.realPath in cmds.file(list=True, q=True))
```

Note that:

- the `filePath` attribute's text from the created mayaUsdProxyShape is not listed out of `_get_all_external_files()`
- `cmds.listAttr(node, usedAsFilename=True)` returns `[u'templatePath', u'iconName']`, which is missing the expected `u'filePath'`
- `cmds.file(list=True, q=True)` does not detect the Sdf Layer path (this may be intended behavior, just figured I'd point it out in case. The other two points are bugs though)

**Expected behavior**
- `_get_all_external_files()` should include the created `tempfile.mkstemp` path in its returned set
- `cmds.listAttr(node, usedAsFilename=True)` should contain `u'filePath'` as a returned value
- (maybe) `cmds.file(list=True, q=True)` would contain the same `tempfile.mkstemp` path as `_get_all_external_files()`

**Specs (if applicable):**
- OS & version: CentOS Linux release 7.9.2009 (Core)
- Compiler & version: 9.3.1
- Maya version: 2022.4.1
- Maya USD commit SHA: 0fbed565b9fae9685f0a6553f3f30135b512463b (v0.12.0)
- Pixar USD commit SHA: [7a5f8c4311fed3ef2271d5e4b51025fb0f513730](https://github.com/PixarAnimationStudios/USD/commit/7a5f8c4311fed3ef2271d5e4b51025fb0f513730) (v21.08)

**Additional context**
Since we're a few versions behind maya-usd's latest release, if you are aware of any changes to the Maya attributes in the plug-in, we can try upgrading to the latest maya-usd if you think that will fix the issue.