Paint callback functionality improvements

[PPakalns]

Support executing additional render passes or adding command buffers from paint callback

[vladbat00]

Hi, thanks for the PR!

Recently, we've also added this node: https://github.com/mvlabat/bevy_egui/blob/3aed12f9292d46c81a1cde1160198c6fb572e253/src/egui_render_to_texture_node.rs

It duplicates probably ~80% of egui_node.rs code, which I'm not a fan of. But until we refactor that, we'll have to introduce any changes to both of them.

[PPakalns]

Logic added, feel free to edit branch if minor fixes are needed.

[PPakalns]

Ping in case of missing this in github feed :)

[vladbat00]

Looks great, thank you!