Add ManageEnv class and refactor stop_env function

A new class called ManageEnv was introduced, encapsulating manager_id, env_id, and a reason. The asynchronous function stop_env in the game_server.py file was refactored to take an instance of this class as an argument instead of three separate arguments. Additionally, a trailing slash was added to the stop_env request URL in the overcooked_gui.py file.