MultiplayerSpawner is broken due to not working with UIDs

[Summersay415]

Tested versions

• Reproducible in latest master (after Expose path properties save UID internally if referencing a resource #97912)
• Not reproducible e.g. in 4.4.dev4

System information

Godot v4.4.dev (ec6a1c0) - Arch Linux #1 SMP PREEMPT_DYNAMIC Fri, 01 Nov 2024 03:30:41 +0000 on Wayland - X11 display driver, Multi-window, 1 monitor - OpenGL 3 (Compatibility) - Mesa Intel(R) Graphics (ADL GT2) - 12th Gen Intel(R) Core(TM) i5-1235U (12 threads)

Issue description

MultiplayerSpawner are broken now because internally they don't work with UIDs (#91084 didn't fix this), and after #97912 paths are stored as UIDs.
Related to: #90903

Steps to reproduce

1. Try to use MultiplayerSpawner in new project, or resave file with it in old project. If it remains from previous Godot versions, it will work.
2. Notice no spawn happening.

Minimal reproduction project (MRP)

bomber.zip

1. Resave world.tscn
2. Run project.
3. Notice no spawn happening.

[Summersay415]

My personal thoughts: revert #97912 and implement #91815, so user will have choice use path or UID, or just fix UIDs in MultiplayerSpawner.
See also: #97912 (comment)
Details on fixing UIDs:
Now it checks for scene_file_path, which remains unchanged on export. But UID cache on export will be updated to remapped path, making converting UID to path in exported game useless, because remapped path and scene_file_path on spawned node won't match. We can implement something like scene_file_uid, but this is another topic for discussion.

[reduz]

UIDs should always be used, if something is not working with UIDs it needs to be fixed to work with it. The roadmap engine wise is that everything moves to UIDs.