fix context publish signature (#3669)

pulsar-client-cpp/python/pulsar/functions/context.py

@@ -124,7 +124,7 @@ class Context(object):
     pass
 
   @abstractmethod
-  def publish(self, topic_name, message, serde_class_name="serde.IdentitySerDe", properties=None, compression_type=None):
+  def publish(self, topic_name, message, serde_class_name="serde.IdentitySerDe", properties=None, compression_type=None, callback=None):
     """Publishes message to topic_name by first serializing the message using serde_class_name serde
     The message will have properties specified if any"""
     pass