Idea: Implement openapi-to-Go client for this library

[lestrrat]

Good

• It's official, slack apparently has an openapi spec https://api.slack.com/specs/openapi/v2/slack_web.json

Bad

• TBH openapi's spec is hard to parse to generate code. It's good for generating docs and what not, but...
• We still need "hints" - for example, we need to map openapi responses to a this library's objects