Implement cancellation notifications (#146)

* Implement cancellation notifications

* Address PR feedback

* Address feedback

src/ModelContextProtocol/Protocol/Messages/CancelledNotification.cs

@@ -0,0 +1,21 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Messages;
+
+/// <summary>
+/// This notification indicates that the result will be unused, so any associated processing SHOULD cease.
+/// </summary>
+public sealed class CancelledNotification
+{
+    /// <summary>
+    /// The ID of the request to cancel.
+    /// </summary>
+    [JsonPropertyName("requestId")]
+    public RequestId RequestId { get; set; }
+
+    /// <summary>
+    /// An optional string describing the reason for the cancellation.
+    /// </summary>
+    [JsonPropertyName("reason")]
+    public string? Reason { get; set; }
+}