Add 3d multiplayer demo

[devmoreir4]

This is a template for a 3D multiplayer game developed in Godot Engine 4.3. It provides a basic structure for a multiplayer setup, where each player has a nickname displayed above their character and the option to choose from four different skins: red, green, blue, or yellow.

Preview:

[Calinou]

I don't think the project added by this PR should be referred to as a template, as we don't expect the demo projects to be used as a base for a full-blown projects. A project template has very different requirements to a demo (and isn't maintained in the same way as a demo). Turning the demos into viable templates would make them a lot more complex, and also require much more strict scrutiny for any changes made to them.

If we want a networked 3D demo, it may be better to modify the existing Platformer 3D or Truck Town demo to add multiplayer to it (similar to what https://github.com/godotengine/tps-demo did at some point).

[devmoreir4]

Thank you for the feedback! I understand the distinction between a demo and a template, and I see how calling this project a "template" may not align with the goals of the repository.

My intention with this PR was to provide the simplest possible 3D multiplayer setup, containing only the essentials so that others could experiment and understand the basic client-server synchronization. The focus is on player movement, animations, and attributes, without adding unnecessary complexity.