Add fetch subscription list

[khmMouna]

No description provided.

[jyaganeh]

one minor comment, but lgtm overall 👍

[jyaganeh]

I think we could maybe do something like this here to avoid creating an empty list, but it's probably not a big deal either way...

subscriptionsPendingResult.AddResultCallback(new ResultCallback((result) 
{
    List<string> list = result switch
    {
        HashSet hashSet => CastHashSetToList(hashSet),
        _ => new List<string>(),
    }

    subscriptions(list);
}