feat: add message to ProgressNotification (#435)

Co-authored-by: ihrpr <inna.hrpr@gmail.com>

src/mcp/types.py

@@ -337,6 +337,11 @@ class ProgressNotificationParams(NotificationParams):
     total is unknown.
     """
     total: float | None = None
+    """
+    Message related to progress. This should provide relevant human readable 
+    progress information.
+    """
+    message: str | None = None
     """Total number of items to process (or total progress required), if known."""
     model_config = ConfigDict(extra="allow")