Adding Heroku deploy button to Bolt Python starter template

[allhaile]

• New sample
• New feature
• Bug fix
• Documentation

Summary

This PR is part of #proj-slack-sample-apps-heroku-deploy-buttons, an initiative to add Heroku one-click deployment buttons across all Slack sample app templates, making it easier for developers to quickly deploy and test Slack apps.

Files added:

• app.json - New Heroku app manifest defining deployment configuration
• Procfile - Process definition for Heroku (worker dyno for socket mode)

Technical details:

• Configured for socket mode deployment using worker dyno (not web dyno)
• Includes all required environment variables for Slack app setup
• Repository URL currently points to personal fork (will update before merge)

Requirements (place an x in each [ ] that applies)

• I’ve checked my submission against the Samples Checklist to ensure it complies with all standards
• 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