Retrieve "additional_properties" from Entity (#48)

* Change model validation to Entity type

* revent to retrieving "additional_properties"

---------

Co-authored-by: Jesus Terrazas <jterrazas@microsoft.com>

Author: JesuTerraz

libraries/microsoft-agents-a365-notifications/microsoft_agents_a365/notifications/models/agent_notification_activity.py

@@ -21,7 +21,7 @@ def __init__(self, activity: Activity):
         entities = self.activity.entities or []
         for ent in entities:
             etype = ent.type.lower()
-            payload = getattr(ent, "properties", ent)
+            payload = getattr(ent, "additional_properties", ent)
 
             if etype == NotificationTypes.EMAIL_NOTIFICATION.lower() and self._email is None:
                 try: