IECoreUSD::ShaderAlgo : Round-trip ColorSpace data on shader parameters.

[boberfly]

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

• Round-trip ColorSpace data on USD attributes, so shaders authored expecting this to work will faithfully translate correctly.

Related Issues

• While doing IECoreMaterialX work, I noticed ColorSpace on shaders don't round-trip.

Dependencies

• N/A

Breaking Changes

• Not sure if it is considered a breaking change, but it does fix up things. Maybe if you are dependent on <paramName>_colorspace parameters existing as real parameters on the USD side, then it'll be problematic but I doubt it would be in-practice. Looking at aiImage they use a dedicated color_space parameter so I doubt it.

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 Cortex project's prevailing coding style and conventions.

I am using the CI to test this one as I've not got Cortex setup in a proper way yet, thanks! :)