MultiplayerSynchronizer - Add ability to sync node references

[nathanfranke]

Describe the project you are working on

RTS with a lot of export nodes to link properties (e.g. the owner of an entity is a player node).

Describe the problem or limitation you are having in your project

I am happily migrating my code to utilize the new export nodes: #1048

However, I still need to keep a setter/getter for the NodePath for when the MultiplayerSynchronizer syncs. Currently, trying to sync the node references directly has no behavior (bug?).

Describe the feature / enhancement and how it helps to overcome the problem or limitation

When Node is serialized to be sent over the network, it should be converted to a NodePath for transit.

#4429 will likely be implemented in two ways:

1. Keep a cache of the previous value and compare that before a synchronization. In this case, the Node reference itself is kept in cache.
2. Use some signal to determine when a property is changed, in which case Nodes in particular won't be very different.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

@export var player: Node

And in the MultiplayerSynchronizer:

If this enhancement will not be used often, can it be worked around with a few lines of script?

@export var player: Node

var player_path: NodePath:
	set(value):
		player = get_node_or_null(value)
	get:
		return player.get_path() if player != null else NodePath()

Issues with this workaround though:

1. Requires another ~5 messy lines of code.
2. For compatibility with MultiplayerSynchronizer - Add an option to sync properties only when they are changed #4429, the workaround will either:
   • Not work at all because player_path's value never actually changes.
   • Work, but be non-optimal because the getter for player_path has a logical branch and calls get_path every time.

Is there a reason why this should be core and not an add-on in the asset library?

MultiplayerSynchronizer is already core