Implement Sequential Hooking and Notification for Multiplayer Button Click

[Liparakis]

Problem

Lack of Interception Logic: There was no built-in mechanism for external addons to react to or intercept the "Start Multiplayer" button click.

Solution

IUiManager.cs: Defined a two-tier API for multiplayer button interactions:

• MultiplayerButtonPressed: A standard C# event for fire-and-forget notifications (logging, non-blocking UI effects).
• RegisterMultiplayerMenuHook: A registration method for "middleware" style hooks. Each hook receives a continuation callback, allowing it to explicitly control when (or if) the menu transition proceeds.

UiManager.cs: Implemented a sequential execution chain (_multiplayerMenuChain).

• Hook Chaining: Each newly registered hook wraps the previous chain, ensuring that hooks execute in sequence (last-registered first).
• Execution Flow: When the button is clicked, the notification event is fired immediately, followed by the first hook in the chain. The core transition to the multiplayer menu (GoToMultiplayerMenu) is only executed at the very end of this chain, provided all hooks have called their respective continuations.

[Extremelyd1]

Thanks for the simple PR!
Couple of easy to click suggestions. The rest looks good.

[Extremelyd1]

Suggested change

	/// Use this for fire-and-forget reactions such as logging or showing a notification?
	/// Use this for fire-and-forget reactions such as logging or showing a notification.

[Extremelyd1]

Suggested change

	_multiplayerMenuChain();
	_multiplayerMenuChain.Invoke();

[Extremelyd1]

I'd prefer explicitly calling .Invoke on Action types to avoid confusion of whether this is an action or a method.