Allow deterministic random output in DialogueManager (for network sync / replay)

[RickyYCheng]

Is your feature request related to a problem? Please describe.

Use cases: multiplayer dialogue sync, save/replay, deterministic testing, replay-from-checkpoint.

In multiplayer game, the [[Hi|Hello|Howdy]] syntax will generate different results on different peer side, since the seed and internal state are different.

For the dialogue_line sync, it is ok since we got line_id: String here to sync. I use rpc to implement simple networking dialogue (For example, cutscenes or real-time dialog).

Describe the solution you'd like
Use RandomNumberGenerator is possible, or just use mutations so that we can do whatever we want.

Describe alternatives you've considered
Call seed() for all peers but it is not good, since it pollutes the global state.

Or use custom functions by using inline mutations so that we can use goto-s based on the result of function. And we can handle states or resolving networking stuffs in (mutation) functions.

Additional context
Non.

[RickyYCheng]

So for [[Hi|Hello|Howdy]], we can use method to substitute:
Nathan: {{option("Hi", "Hello", "Howdy")}}, this is some dialogue.

class Utils extends RefCounted:
	static var rng := RandomNumberGenerator.new()
	static func incr(state: Dictionary) -> void:
		if state.has("foo"):
			state.foo += 1
	static func decr(state: Dictionary) -> void:
		if state.has("foo"):
			state.foo -= 1
	static func option(...args: Array) -> Variant:
		var idx := rng.randi_range(0, args.size() - 1)
		return args[idx]

@rpc("authority", "call_remote", "reliable")
func _start_balloon(_seed: int) -> void:
	Utils.rng.seed = _seed
	
	balloon.start(dialog, "start", [
		{"state": state},
		Utils,
	])

And for multiline randomization, probably we could use match clauses.