Allow user to write custom Shader for custom Shading Model

[TheOmnisimush]

Issue description: Currently Flax allows user to write Custom Shader for PostProcess, Compute Shader. It will be great if Flax allows user to write their own Shading Model, so users do not have to depend on Flax Devs when they need something different from the built-in features.

• One of the reason I chose Unity over Unreal Engine (before the whole debacle) was because Unity allows me to write my own shader from scratch So if I don't like something from Unity built-in (either if because it's too slow in performance or simply the shader looks bad) I can write my own shader without bothering the devs to add this or that in the forum, since we can not make everyone happy I'd rather have the devs spend those time developing other more important features for the engine.

• It will also help to reduce "suggestion issues" about Shader , like this one for example. https://github.com/FlaxEngine/FlaxEngine/issues/1492 Although I agree that this will NOT fix ALL issues regarding Shaders, but at least for Graphic Programmers like me I can make an effort to get the looks I desire before bothering the devs.

Flax version: 1.6.6344

[HydrogenC]

As a workaround, maybe add more custom control such as allow specifying custom stencil operations, depth methods and add more blend options in the material editor (like unity). This might simplify the creation of some vfx.