Allow calling add_render_graph_node on World.

[andriyDev]

Objective

• This unblocks some work I am doing for Use RenderStartup instead of a FromWorld impl whenever possible #19887.

Solution

• Rename RenderGraphApp to RenderGraphExt.
• Implement RenderGraphExt for World.
• Change SubApp and App to call the World impl.

[IceSentry]

Can you explain why it unblocks you? I'm not sure I understand why this change is important

[andriyDev]

@IceSentry It's not strictly necessary but it removes a bunch of duplicate code.

I want to add graph nodes in RenderStartup only if the RenderDevice has certain supported features. Since we're in a system, we don't have access to App. I also don't want render graph nodes sitting around doing nothing at best, or blocking things at worst.

Technically I could use the raw RenderGraph API, but that would just mean duplicating all the code here. It's better to just allow us to add graph nodes with World directly the same way we add them to an app.