addPendingAck method
Adds a pending acknowledgment
Implementation
void addPendingAck(PendingAck ack) {
if (ack.ackId == null) return; // No ack required
_pendingAcks[ack.ackId!] = ack;
final subscriptionAcks = _subscriptionAcks.putIfAbsent(ack.subscriptionId, () => <PendingAck>[]);
subscriptionAcks.add(ack);
}