Improve progress reporting (#145)

* Improve progress reporting

- Enable injecting an `IProgress<>` into a tool call to report progress when the client has supplied a progress token
- Add a custom type for ProgressToken and ProgressNotification to map to the schema
- Simplify RequestId to match ProgressToken's shape
- Serialize StdioClientTransport's SendMessageAsync implementation
- Add strongly-typed names for all request methods
- Change all request IDs created by McpJsonRpcEndpoint to include a guid for that endpoint

* Address PR feedback

src/ModelContextProtocol/Protocol/Messages/NotificationMethods.cs

@@ -34,4 +34,27 @@ public static class NotificationMethods
     /// Sent by the server when a log message is generated.
     /// </summary>
     public const string LoggingMessageNotification = "notifications/message";
+
+    /// <summary>
+    /// Sent from the client to the server after initialization has finished.
+    /// </summary>
+    public const string InitializedNotification = "notifications/initialized";
+
+    /// <summary>
+    /// Sent to inform the receiver of a progress update for a long-running request.
+    /// </summary>
+    public const string ProgressNotification = "notifications/progress";
+
+    /// <summary>
+    /// Sent by either side to indicate that it is cancelling a previously-issued request.
+    /// </summary>
+    /// <remarks>
+    /// The request SHOULD still be in-flight, but due to communication latency, it is always possible that this notification
+    /// MAY arrive after the request has already finished.
+    /// 
+    /// This notification indicates that the result will be unused, so any associated processing SHOULD cease.
+    /// 
+    /// A client MUST NOT attempt to cancel its `initialize` request.".
+    /// </remarks>
+    public const string CancelledNotification = "notifications/cancelled";
 }