Restrict bot usage to specific channels or users

[PeterDaveHello]

Related to #32

Add access control feature to restrict bot usage to specific channels or users.

• Environment Variables: Add ALLOWED_CHANNELS and ALLOWED_USERS environment variables in main.py and main_prod.py to specify allowed channels and users.
• Middleware: Update middleware in main.py and main_prod.py to check if the bot is allowed in the channel or by the user.
• Access Control Function: Add is_bot_allowed function in app/bolt_listeners.py to check if the bot is allowed to respond based on the specified channels and users.
• Event Handlers: Update respond_to_app_mention and respond_to_new_message in app/bolt_listeners.py to use is_bot_allowed function.
• Documentation: Update README.md to include instructions on how to configure the ALLOWED_CHANNELS and ALLOWED_USERS environment variables.

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For more details, open the Copilot Workspace session.