- Built-in slot types for Lex utterances: https://docs.aws.amazon.com/lex/latest/dg/howitworks-builtins-slots.html
- Lambda input events and response types: https://docs.aws.amazon.com/lex/latest/dg/lambda-input-response-format.html
- Responding with the Slack challenge value: https://api.slack.com/events/url_verification
- A detailed guide on bot users: https://api.slack.com/bot-users