Allow Custom File Extensions for Resources (RES_BASE_EXTENSION macro)

### Godot version

4.x

### godot-cpp version

master

### System information

Windows 10 i7-6800K

### Issue description

I am currently porting my terrain system for godot 4 from module to extension and visually having custom resource names inside the editor and the file system without having to write a custom ResourceFormatLoader/Saver helps a lot.


Adding custom file extensions to a resource is not much of an important feature, but it is very useful one to avoid the clutter of .res or .tres names

### Steps to reproduce

Create a Custom resource class. There is no method or macro available to set the resource save extension

### Minimal reproduction project

not neccesary