.Net The 'Source' property added to the ChatMessageContent class (#5091)

### Motivation and Context  
For agent customization story, when converting or translating messages
from one agent to another, we need a way to access the agent that
generated the messages.
   
### Description  
This PR adds the 'Source' property to the ChatMessageContent class.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/SemanticKernel.Abstractions/Contents/ChatMessageContent.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Text;
 using System.Text.Json.Serialization;
 using Microsoft.SemanticKernel.ChatCompletion;
@@ -33,6 +34,17 @@ public class ChatMessageContent : KernelContent
     [JsonIgnore]
     public Encoding Encoding { get; set; }
 
+    /// <summary>
+    /// Represents the source of the message.
+    /// </summary>
+    /// <remarks>
+    /// The source is corresponds to the entity that generated this message.
+    /// The property is intended to be used by agents to associate themselves with the messages they generate.
+    /// </remarks>
+    [Experimental("SKEXP0101")]
+    [JsonIgnore]
+    public object? Source { get; set; }
+
     /// <summary>
     /// Creates a new instance of the <see cref="ChatMessageContent"/> class
     /// </summary>