Autodesk: Unit test support for HgiVulkan

[erikaharrison-adsk]

Description of Change(s)

A HGI unit test helper was added to the HdSt project to facilitate unit testing of HGI backends.
A unit test that tests the following has been added as well

• Testing of successful HgiVulkan creation (through HGI abstraction)
• Testing of successful sample HgiVulkan graphics pipeline (through HGI abstraction)
• Testing of successful sample graphics queue with simple triangle draw execution (through HGI abstraction)
• Provided image base-line comparison with unit test output written to disk (following a GPU - CPU copy)

Note: This PR is child of #2553. Please refer it for more context.

Fixes Issue(s)

• N/A

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8837