Forward declarations of godot-cpp classes.

### Godot version

v4.1.1.stable.custom_build [bd6af8e0e]

### godot-cpp version

4.1 4eed2d7be017ef06521665604990114c9fff7c77

### System information

Godot v4.1.1.stable (bd6af8e0e) - Linux Mint 21.2 (Victoria) - Vulkan (Forward+) - integrated Intel(R) Graphics (RPL-P) () - 13th Gen Intel(R) Core(TM) i7-1360P (16 Threads)

### Issue description

When declaring, in a header file, a class member that is a pointer to a godot class, e.g. `::godot::Node * pointerToNode`, I need to either `#include ` or add something like

namespace godot
{
class Node;
}

I prefer the latter, because the `#include` slows down compilation. A godot header file with forward declarations for all godot classes would be nice.

I previously asked a question here: https://godotforums.org/d/36916-c-forward-declarations-for-godot-classes

### Steps to reproduce

I hope you don't mind asking for this enhancement here, I thought of adding it to the godot repository itself, but it seems to me this issue is very much related to godot-cpp. Please let me know if I should file this issue elsewhere.

### Minimal reproduction project

Any project will do.