Import userProperties with MayaUSDImport

**Is your feature request related to a problem? Please describe.**

I have a Cube prim with a user property string attribute, for example:

```
custom string userProperties:customId = "hello:world"
```

When importing this with `mayaUSDImport` the attribute does not appear on the mesh or transform.

It's unclear to me what flag I should be passing to import the user properties as custom attributes on the import.

**Describe the solution you'd like**

Either default behavior should import the user properties or a flag `importUserProperties` would be added to enable it.

**Describe alternatives you've considered**

A python chaser could be implemented - however as far as I know it'd be non-trivial to generate the correct attribute type with the valid time-samples, etc. - which could be trivialized with [UsdMayaReadUtil::ReadUsdAttribute()](https://github.com/Autodesk/maya-usd/blob/3f5039ceba21ad63c5447f2245c222c87959459c/lib/mayaUsd/fileio/utils/readUtil.h#L203) exposed to Python API as came up as a request [here](https://github.com/Autodesk/maya-usd/issues/3313#issuecomment-1716966019).

**Additional context**

The same 'issue' is also present in "Edit as Maya data" where it does not expose the user properties to the user for altering values.

Interestingly enough user properties are exposed however in a Maya USD Proxy with the UFE selection, e.g.:


Start at the mayaUSDImport user-property handling and inspect lib/mayaUsd/fileio/utils/readUtil.h, especially UsdMayaReadUtil::ReadUsdAttribute(). Compare this with the Python chaser alternative described in the issue. Done means userProperties are available on the imported mesh or transform, either by default or through an importUserProperties flag, with attribute types and time samples preserved.

Written by the indexing model from the issue text.