feat: support configurable game server address and port

* Launch game servers in headless mode
* Add configurable game address and port to GameData
* Pass dynamically assigned server ports to clients
* Update lobby defaults for `0.0.0.0:8080`
* Refactor server command-line argument handling

globals/instance_launcher.gd

@@ -13,7 +13,10 @@ func _wait_for_child(id: String, port: String, mode: String) -> void:
 
 	var output    := []
 	var exit_code := OS.execute(
-		"./game.sh", ["--", "server_" + port, "game", mode],
+		"./game.sh",
+		[
+			"--headless", "--", "server_" + port, "game", mode
+		],
 		output,
 		true
 	)