Support more properties in `User`

[nlopes]

i also see there's a ton more properties we don't support. Maybe I will create a PR?

See exhaustive list here: https://api.slack.com/types/user

Yeah, we tend not to add everything in the sample responses because sometimes those are actually wrong. So I'd rather not just take these and add.

I'm going to check what Slack has in their official JS/Python/Java packages and see what we're missing and port. That's usually much more high fidelity.

Originally posted by @nlopes in https://github.com/slack-go/slack/discussions/1455#discussioncomment-13664955