[fresh_graphql] Link for Subscription

Great idea this package and exactly what I was looking for. 🙇
Based on this idea it would be possible to create a similar link implementation for subscription.

What do you think?

This is my current code that could be replaced
```
class WSLink extends Link {
/// Creates a new [WebSocketLink] instance with the specified config.
WSLink(
this.url, {
this.headers = const {},
this.getToken,
});

final String url;
final Map headers;
final Future? getToken;

// cannot be final because we're changing the instance upon a header change.
SocketClient? _socketClient;

@override
Stream request(Request request, [forward]) async* {
// Get Token or Refresh
if (getToken != null) {
var token = await getToken;
headers['Authorization'] = 'Bearer $token';
}

if (_socketClient == null) {
connectOrReconnect();
}

yield* _socketClient!.subscribe(request, true);
}

/// Connects or reconnects to the server with the specified headers.
void connectOrReconnect() {
_socketClient?.dispose();
_socketClient = SocketClient(
url,
config: SocketClientConfig(
inactivityTimeout: Duration(seconds: 30),
delayBetweenReconnectionAttempts: const Duration(seconds: 2),
autoReconnect: true,
connect: (url, protocols) => IOWebSocketChannel.connect(
url,
protocols: protocols,
headers: headers,
),
),
);
}

/// Disposes the underlying socket client explicitly. Only use this, if you want to disconnect from
/// the current server in favour of another one. If that's the case, create a new [WebSocketLink] instance.
Future dispose() async {
await _socketClient?.dispose();
_socketClient = null;
}
}
```