.NET: [Feature Branch] Add basic durable workflow support (#3648)

* Add basic durable workflow support.

* PR feedback fixes

* Add conditional edge sample.

* PR feedback fixes.

* Minor cleanup.

* Minor cleanup

* Minor formatting improvements.

* Improve comments/documentation on the execution flow.

dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/DurableActivityOutput.cs

@@ -0,0 +1,19 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.DurableTask.Workflows;
+
+/// <summary>
+/// Output payload from activity execution, containing the result and other metadata.
+/// </summary>
+internal sealed class DurableActivityOutput
+{
+    /// <summary>
+    /// Gets or sets the serialized result of the activity.
+    /// </summary>
+    public string? Result { get; set; }
+
+    /// <summary>
+    /// Gets or sets the collection of messages that have been sent.
+    /// </summary>
+    public List<SentMessageInfo> SentMessages { get; set; } = [];
+}