Support custom implicit shapes

[sebastienblor]

We currently support custom procedurals by using a ArnoldProceduralCustom schema.
We would want to apply a similar logic for implicit nodes. In a first step we could use the same ArnoldProceduralCustom schema, as it's already properly supported in hydra

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-17629