Support user whitelisting

[PercussiveElbow]

Useful if you don't want everyone to be able to interact with the bot.

[codingjoe]

Hi there 👋,

Thanks for reaching out and your submission. The implementation looks pretty clean, but please still consider starting a discussion or issue next before a PR to smoothen the process.

Generally, this feature looks very Slack-specific. I feel like the user management should be happening inside Slack, not here. There might already be a way to do this. Have you looked into this and maybe mind sharing your findings?

Other than that, this should be SLACK_ALLOWED_USERS with a prefix, and we try to avoid black-and-white terms largely.

Best
Joe