fix: Adds back socket connected event (#1688)

### Summary
Adds back the `SocketConnected` event. This event only fires asynchronously! (TcpServer thread).

Example:
```cs

public static void Configure()
{
    TcpServer.EventSink.SocketConnected += OnSocketConnected;
}

// WARNING: Executed on the TcpServer thread!
private static void OnSocketConnected(TcpServer.SocketConnectedEventArgs args)
{
    if (... logic here...)
    {
        AdminFirewall.Add(((IPEndPoint)Socket.RemoteEndPoint)!.Address);
    }
}
```

Projects/Server/Network/TcpServer.cs

@@ -256,7 +256,15 @@ private static void ProcessConnection(Socket socket)
                 return;
             }
 
-            if (Firewall.IsBlocked(remoteIP))
+            var firewalled = Firewall.IsBlocked(remoteIP);
+            if (!firewalled)
+            {
+                var socketConnectedArgs = new SocketConnectedEventArgs(socket);
+                EventSink.InvokeSocketConnected(socketConnectedArgs);
+                firewalled = !socketConnectedArgs.ConnectionAllowed;
+            }
+
+            if (firewalled)
             {
                 TraceDisconnect("Firewalled", remoteIP);
                 logger.Debug("{Address} Firewalled", remoteIP);
@@ -292,4 +300,22 @@ private static void TraceDisconnect(string reason, IPAddress ip)
             // ignored
         }
     }
+
+    public static class EventSink
+    {
+        // IMPORTANT: This is executed asynchronously! Do not run any game thread code on these delegates!
+        public static event Action<SocketConnectedEventArgs> SocketConnected;
+
+        internal static void InvokeSocketConnected(SocketConnectedEventArgs context) =>
+            SocketConnected?.Invoke(context);
+    }
+
+    public class SocketConnectedEventArgs
+    {
+        public Socket Socket { get; }
+
+        public bool ConnectionAllowed { get; set; } = true;
+
+        internal SocketConnectedEventArgs(Socket socket) => Socket = socket;
+    }
 }