Paint callback support

[PPakalns]

Resolves #294

Bevy render graph nodes create render passes, paint callback can not be expanded into a render graph subgraph.

Graph node can contain custom rendering code or, for example, Render Phase abstraction to render multiple type of resources with multiple render pipelines in the current render pass.
This implementation supports bevy rendering logic with custom render implementation.

For now native wgpu RenderPass can not be used because of the following issue: bevyengine/bevy#13225

[vladbat00]

Awesome work, thank you! I'm also amazed how it just works in browsers

[vladbat00]

I pushed a couple of fixes to make CI happy and also did a small and a bit of unnecessary refactor, I hope it doesn't break anything :D

[vladbat00]

I've just found a copy-paste error that I added: will be fixed in #304 if I manage to implement callbacks support for worldspace UI based on the current implementation (or separately).