Improved shader graph support

Describe the problem

Graphics Tools supports Unity's Universal Render Pipeline (URP) but does not expose functionality to the new Shader Graph that comes with URP.

Describe the solution you'd like

Graphics Tools should expose a library of common functions for effects it supports via custom shader graph nodes. This will allow users to make more bespoke shaders and effects.

Describe alternatives you've considered

Not use Shader Graph and continue to use the uber shader approach.

Additional context

See Graphics Tools for Unreal for example of how methods are exposed to a shader graph: https://github.com/microsoft/MixedReality-GraphicsTools-Unreal

Inizia dalla documentazione di Unity Shader Graph Custom Function Node e dall’esempio di Graphics Tools for Unreal collegato nell’issue. Usali per identificare quali effetti esistenti di Graphics Tools dovrebbero essere esposti per URP Shader Graph. Il lavoro è completo quando gli utenti possono accedere a una libreria di effetti comuni supportati tramite custom nodes per shader più personalizzati.