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.

[seratch]

Thank you for sending this PR. I apologize for my slow response. Access control could indeed be useful for enterprise use cases, but there are many ways people might want to configure it (e.g., managing the list in a database, workspace level for an enterprise grid org, etc.). Thus, this environment variable approach may not meet everyone's needs. Please consider forking this project or referring to this repo as a Git submodule and implementing your own global middleware and main.py.

[seratch]

In #32, I said "Happy with having some env variables to customize main.py for the use case described here." but while checking your code, I changed my mind. I'm sorry about that.