Iterator based subscription API (DEV-112)

[hunter1703]

Added : Subscription API similar to read API i.e. iteration based rather than callback based
Added : With new API, user code can now receive control messages like Checkpoint, CaughtUp (this new control message will be part of a separate PR and is the actual goal of DB-224) etc. from the subscription in addition to events

Example :

var subscription = client.SubscribeToAll(FromAll.Start);

await foreach (var message in subscription.Messages) {
    if (message is StreamMessage.Event eventMessage) {
        await eventAppeared(eventMessage.ResolvedEvent);
    } else if (message is StreamMessage.SubscriptionMessage.Checkpoint checkpointMessage) {
        await checkpointReached(checkpointMessage.Position);
    }
}

subscriptionDropped();

[linear]

DB-224 Support notifying catchup subscriptions that they are live

[linear]

DEV-106 Support notifying catchup subscriptions that they are live

Client side part of DB-224

[linear]

DB-224 Support notifying catchup subscriptions that they are live

DEV-112 Subscription is live message support in .NET client

[RagingKore]

Added to latest version.