Add FutureSignal that wraps futures

[jinyus]

Re: #6

This class makes it straightforward to reset the state of the signal.

Usage:

fetchData() {
  return 'abc';
}

final futureSignal = FutureSignal<String>(
  () => Future.delayed(const Duration(seconds: 1), () => fetchData()),
);

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  void _reloadData() {
    futureSignal.reset();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Builder(builder: (context) {
              final state = futureSignal.watch(context);
              final text = switch (state) {
                SignalError(value: var v) => 'Error: $v',
                SignalValue(value: var v) => 'Success: $v',
                _ => 'Loading...',
              };
              return Text(
                text,
                style: Theme.of(context).textTheme.headlineMedium!,
              );
            }),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _reloadData,
        child: const Icon(Icons.refresh),
      ),
    );
  }
}

[rodydavis]

LGTM

[rodydavis]

I am going to add back the timeout, and add the same for Stream. But LGTM!