Simplify OSL shader generator for surfaceshader output

[ld-kerley]

Currently if the OSL shader generator is passed an element that outputs a single surfaceshader type, then it injects the shader code from the OSL implementation of surfacematerial node. Because the surfaceshader type is a struct and it needs to be resolved down to a closure color for output.

This PR modifies the shader generator to insert an instance of the surfacematerial node, instead of hardcoding the shader source in the shader generator. This decouples the library, and means we just have a single source of truth for the shader code in the surfacematerial node for OSL.

Adds a new API call on the ShaderGraph node to insert a node upstream of an output of the graph.

[niklasharrysson]

This looks good to me, thank @ld-kerley

[jstone-lucasfilm]

Other than the minor notes I've added inline, this change looks good to me.

I'd probably recommend that we hold this change in reserve for 1.39.5, since it's a substantial change in logic.