feat: implement jetstream pull consumer fetch

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

@@ -0,0 +1,35 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream.Consumers;
+
+public sealed class PullConsumerEngine
+{
+    public async ValueTask<PullFetchBatch> FetchAsync(StreamHandle stream, ConsumerHandle consumer, int batch, CancellationToken ct)
+    {
+        var messages = new List<StoredMessage>(batch);
+        var sequence = consumer.NextSequence;
+
+        for (var i = 0; i < batch; i++)
+        {
+            var message = await stream.Store.LoadAsync(sequence, ct);
+            if (message == null)
+                break;
+
+            messages.Add(message);
+            sequence++;
+        }
+
+        consumer.NextSequence = sequence;
+        return new PullFetchBatch(messages);
+    }
+}
+
+public sealed class PullFetchBatch
+{
+    public IReadOnlyList<StoredMessage> Messages { get; }
+
+    public PullFetchBatch(IReadOnlyList<StoredMessage> messages)
+    {
+        Messages = messages;
+    }
+}