Autodesk: Provided initialization hgi backend selection for render plugin.

[erikaharrison-adsk]

Description of Change(s)

This PR provides the ability to select Vulkan/OpenGL as a Hgi backend from the render delegate and plugin's perspective.

Fixes Issue(s)

• N/A

• [X] I have verified that all unit tests pass with the proposed changes

• [X] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9058