zulip_bots: Add a boilerplate bot for external bots.

Add packaged_helloworld as an example of a PyPI package setup for
an external zulip bot that can be installed via pip and lanuched
without the need to include it in the zulip_bots/bots directory.

packaged_helloworld/README.md

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+This is a boilerplate package for a Zulip bot that can be installed from pip
+and launched using the `zulip-run-bots` command.

packaged_helloworld/packaged_helloworld/__init__.py

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+__version__ = "1.0.0"

packaged_helloworld/packaged_helloworld/doc.md

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+Simple Zulip bot that will respond to any query with a "beep boop".
+
+The packaged_helloworld bot is a boilerplate bot that can be used as a
+template for more sophisticated/evolved Zulip bots that can be
+installed separately.

packaged_helloworld/packaged_helloworld/packaged_helloworld.py

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+# See readme.md for instructions on running this code.
+from typing import Any, Dict
+
+import packaged_helloworld
+
+from zulip_bots.lib import BotHandler
+
+__version__ = packaged_helloworld.__version__
+
+
+class HelloWorldHandler:
+    def usage(self) -> str:
+        return """
+        This is a boilerplate bot that responds to a user query with
+        "beep boop", which is robot for "Hello World".
+
+        This bot can be used as a template for other, more
+        sophisticated, bots that can be installed separately.
+        """
+
+    def handle_message(self, message: Dict[str, Any], bot_handler: BotHandler) -> None:
+        content = "beep boop"  # type: str
+        bot_handler.send_reply(message, content)
+
+        emoji_name = "wave"  # type: str
+        bot_handler.react(message, emoji_name)
+        return
+
+
+handler_class = HelloWorldHandler

packaged_helloworld/setup.py

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+import packaged_helloworld
+from setuptools import find_packages, setup
+
+package_info = {
+    "name": "packaged_helloworld",
+    "version": packaged_helloworld.__version__,
+    "entry_points": {
+        "zulip_bots.registry": ["packaged_helloworld=packaged_helloworld.packaged_helloworld"],
+    },
+    "packages": find_packages(),
+}
+
+setup(**package_info)