bots: Find external packaged bots via 'zulip_bots.registry' entry_point.

Now we will be able to execute `zulip-run-bot` with the `-r` argument
to search for and run bots from the `zulip_bots.registry` entry_point.

Each entry point should have the name correspond to the bot name,
and have the value be the bot module. E.g, an Python package for a
bot called "packaged_bot" should have an `entry_points` setup like
the following:

setup(
    ...
    entry_points={
        "zulip_bot.registry":[
            "packaged_bot=packaged_bot.packaged_bot"
        ]
    }
    ...
)

whose file structure may look like this:

packaged_bot/
├───packaged_bot/
|   ├───packaged_bot.py  # The bot module
|   ├───test_packaged_bot.py
|   ├───packaged_bot.conf
|   └───doc.md
└───setup.py  # Register the entry points here

Add test case.

zulip_bots/setup.py

@@ -66,6 +66,7 @@ setuptools_info = dict(
         "lxml",
         "BeautifulSoup4",
         "typing_extensions",
+        'importlib-metadata >= 3.6; python_version  < "3.10"',
     ],
 )