USDShader : Shader type namespacing and assignment improvements

[boberfly]

Generally describe what this PR will do, and why it is needed

• USDShader :
  • A namespace prefix is now set for the shader type from the USD source type, except for USD and glslfx built-ins which correspond to USDLux lights and USDPreviewSurface built-ins
  • Added a way to register namespace overrides from USD shader source types if the resulting USD source type is undesirable
  • The correct shader assignment for displacement and volume shaders are now set for shaders which have a context of displacement or volume

I put this as draft, I am not sure if we want a register or if there's a better way to figure out shader namespacing for shader assignments.

Related issues

• NA

Dependencies

• MaterialX but it should be available (need to check the CI)

Breaking changes

• Added registerShaderNameSpace to USDShader - let me know if this should target main/1.5 instead of 1.4

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.