usdShade: Add ShaderValidator

[beersandrew]

Description of Change(s)

• Added equivalent validation checks from ARShaderChecker in the new ValidationFramework, including validation implementation code, test code, tokens, and an entry in plugInfo.json

Link to proposal (if applicable)

Fixes Issue(s)

• https://github.com/orgs/PixarAnimationStudios/projects/3/views/1?pane=issue&itemId=65424945

Checklist

[X] I have created this PR based on the dev branch

[X] I have followed the coding conventions

[X] I have added unit tests that exercise this functionality (Reference: testing guidelines)

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

[X] I have submitted a signed Contributor License Agreement (Reference: Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10396

[jesschimein]

/AzurePipelines run

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