allow for custom pxr namespace

[pmolodo]

Description (this won't be part of the changelog)

Namespace "pxr::" was hardcoded in one spot, which broke our build when we switched to using our own private USD namesapce

Changelog

• use PXR_NS instead of "pxr" in ProxyShape.cpp

Checklist (Please do not remove this line)

• Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• Do any added files have the correct AL Apache Licence Header?
• Are there Doxygen comments in the headers?
• Are any new features, behaviour changes documented in the .md format documentation?
• Have you added, updated tests to cover new features and behaviour changes?
• Have you filled out at least one changelog entry?