Support per-channel connections in arnold shaders (procedural)

[sebastienblor]

Describe the bug Arnold supports connecting a shader to an attribute channel, e.g.

standard_surface.base_color.r = float_ramp

We'd need to add support for this in the procedural see #166 for the equivalent support in the render delegate

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13232