Function onTokenChanged

onTokenChanged(
    appCheckInstance: AppCheck,
    observer: PartialObserver<AppCheckTokenResult>,
): Unsubscribe

onTokenChanged(
    appCheckInstance: AppCheck,
    onNext: (tokenResult: AppCheckTokenResult) => void,
    onError?: (error: Error) => void,
    onCompletion?: () => void,
): Unsubscribe