feat: implement jetstream push delivery and heartbeat

src/NATS.Server/JetStream/Consumers/PushConsumerEngine.cs

@@ -0,0 +1,31 @@
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Consumers;
+
+public sealed class PushConsumerEngine
+{
+    public void Enqueue(Queue<PushFrame> queue, StoredMessage message, ConsumerConfig config)
+    {
+        queue.Enqueue(new PushFrame
+        {
+            IsData = true,
+            Message = message,
+        });
+
+        if (config.HeartbeatMs > 0)
+        {
+            queue.Enqueue(new PushFrame
+            {
+                IsHeartbeat = true,
+            });
+        }
+    }
+}
+
+public sealed class PushFrame
+{
+    public bool IsData { get; init; }
+    public bool IsHeartbeat { get; init; }
+    public StoredMessage? Message { get; init; }
+}