Restrictive bot access

[noplanman]

(From php-telegram-bot/core#527)

Would be cool to implement this as an ultimate firewall, before the bot code is even touched.
Although, it could also be cool to have it just after initialisation, so that rules could be configured via commands.

Hmmm.... 🤔

@jacklul Ideas very welcome as always!

[jacklul]

I guess simple access filtering by basic fields like chat and user ID should be fine, also the maintenance mode letting admins still use the bot and other users not.