.NET: [BREAKING] Add ability to mark the source of Agent request messages and use that for filtering (#3540)

* Add ability to mark the source of Agent request messages and use that for filtering

* Add support for source, in addition to source type, and add unit tests for automatic stamping

* Address PR comments.

* Add merge fixes

* Address PR comments

dotnet/src/Microsoft.Agents.AI.Abstractions/ChatMessageExtensions.cs

@@ -0,0 +1,27 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI;
+
+/// <summary>
+/// Contains extension methods for <see cref="ChatMessage"/>
+/// </summary>
+public static class ChatMessageExtensions
+{
+    /// <summary>
+    /// Gets the source of the provided <see cref="ChatMessage"/> in the context of messages passed into an agent run.
+    /// </summary>
+    /// <param name="message">The <see cref="ChatMessage"/> for which we need the source.</param>
+    /// <returns>An <see cref="AgentRequestMessageSourceType"/> value indicating the source of the <see cref="ChatMessage"/>. Defaults to <see
+    /// cref="AgentRequestMessageSourceType.External"/> if no explicit source is defined.</returns>
+    public static AgentRequestMessageSourceType GetAgentRequestMessageSource(this ChatMessage message)
+    {
+        if (message.AdditionalProperties?.TryGetValue(AgentRequestMessageSourceType.AdditionalPropertiesKey, out var source) is true && source is AgentRequestMessageSourceType typedSource)
+        {
+            return typedSource;
+        }
+
+        return AgentRequestMessageSourceType.External;
+    }
+}