Describe the project you are working on:
I'm implementing an Entity Component System module for Godot 4.0. To properly integrate it, it's necessary to process the servers and the other Godot parts (like the SceneTree) in a specific way.

Describe the problem or limitation you are having in your project:
To customize the process loop, so to use the ECS pipeline, it's necessary to change the Main::iteration. Right now it's not possible customize the process loop at such deepness without forking Godot.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
The idea is to create a system that allows, from a module, to override the Main::iteration function and so be able to customize it. In this way, it's possible to add features that need to work at such deepness with the engine, (like for the ECS module), without forking or requesting feature proposal. Features that can be shipped separately from the engine, but still well integrated and easy to use.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
It's possible to see the needed change directly into this PR: godotengine/godot#42875

If this enhancement will not be used often, can it be worked around with a few lines of script?:
It's a low level change.

Is there a reason why this should be core and not an add-on in the asset library?:
It's a low level change.