bots: Support adding reactions to message for a bot.

It adds a react() function that allows a bot to react to a message in lib.py.
It adds an example of the use of react() function and its test.

The changes are in the following files:
  - lib.py
  - helloworld.py
  - tests/test_lib.py
  - test_lib.py

zulip_bots/zulip_bots/test_lib.py

@@ -50,6 +50,9 @@ class StubBotHandler:
         self.transcript.append(('send_reply', response_message))
         return self.message_server.send(response_message)
 
+    def react(self, message: Dict[str, Any], emoji_name: str) -> Dict[str, Any]:
+        return self.message_server.add_reaction(emoji_name)
+
     def update_message(self, message: Dict[str, Any]) -> None:
         self.message_server.update(message)