feat: change downloadUrl to future

[amondnet]

If the downloadUrl is future, it will be helpful when implementing other package stores.

For example, I am using gcloud for package store.

  @override
  FutureOr<String> downloadUrl(String name, String version) async {
    var object = await _bucket.info('$name-$version.tar.gz');
    final url = object.downloadLink.toString();
    return url;
  }

[danilofuchs]

I recommend using FutureOr<String> instead of Future<String> so it is more flexible and does not break existing implementations