Cycles ShaderNetworkAlgo : Add support for normal imputs for USDPreviewSurface

[boberfly]

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

• Now that Cycles provides tangents, bring in support for normal inputs into USDPreviewSurface by adding in a normal_map node and vector_math to normalize. ALab tends to output scale/bias with negative values and will hard-crash Cycles (I wonder if our multiply_add approach to scale/bias textures needs use_clamp enabled?). Should I put preprocessor defines in here for older Cycles versions that don't generate tangents automatically to not load in normals?

Related issues

• NA

Dependencies

• NA

Breaking changes

• Normal maps or normal inputs will now appear (if any).

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.