Add IMessagesRetrievableThread interface (#63)

* Add a MessageAgentThread and GetService for Thread

* Remove redundant check and experimental attribuet.

* Address PR comments.

* Fix file name.

* Update comments.

* Remove GetService

dotnet/src/Microsoft.Agents.Abstractions/IMessagesRetrievableThread.cs

@@ -0,0 +1,37 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents;
+
+/// <summary>
+/// An interface for agent threads that allow retrieval of messages in the thread for agent invocation.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Some agents need to be invoked with all relevant chat history messages in order to produce a result, while some must be invoked
+/// with the id of a server side thread that contains the chat history.
+/// </para>
+/// <para>
+/// This interface can be implemented by all thread types that support the case where the agent is invoked with the chat history.
+/// Implementations must consider the size of the messages provided, so that they do not exceed the maximum size of the context window
+/// of the agent they are used with. Where appropriate, implementations should truncate or summarize messages so that the size of messages
+/// are constrained.
+/// </para>
+/// </remarks>
+public interface IMessagesRetrievableThread
+{
+    /// <summary>
+    /// Asynchronously retrieves all messages to be used for the agent invocation.
+    /// </summary>
+    /// <remarks>
+    /// Messages are returned in ascending chronological order.
+    /// </remarks>
+    /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
+    /// <returns>The messages in the thread.</returns>
+    /// <exception cref="InvalidOperationException">The thread has been deleted.</exception>
+    IAsyncEnumerable<ChatMessage> GetMessagesAsync(CancellationToken cancellationToken = default);
+}