feat: add requirements.txt

[cchensh]

Type of change

• New feature
• Bug fix
• Documentation

Summary

This PR adds requirements.txt file for Slack CLI to recognize it as Bolt app

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

[mwbrooks]

✅ Thanks @cchensh

🧪 I was able to successfully create a new app from this template using the Slack CLI and confirmed that the "source": "remote" is set.

$ slack create -t https://github.com/slack-samples/bolt-python-search-template -b cchen-requirements

$ cat beautiful-groundhog-833/.slack/config.json
{
  "manifest": {
    "source": "remote"
  },
  "project_id": "..."
}

I was able to install the dependencies and run the project. 👌🏻