Investigate replacement of ArnoldNodeGraph with builtin node graph

[sebastienblor]

In order to support shader connections to all types of nodes, we currently rely on ArnoldNodeGraph primitives.
We should investigate if we can replace this with builtin NodeGraph which should be enough in theory.

Also, currently these node graphs are referenced by strings (.e.g a light attribute light:shaders is set to the node graph prim name as a string), which is a problem if the usd file is loaded by another layer inside of a hierarchy. The full path of the node graph is no longer the same in that situation and we cannot resolve it. A Relationship attribute would solve this

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15289