Add JSON encoding to all applicable Web API arguments

[jmanian]

There was at least one chat.* method that was missing the auto-encoding of blocks and attachments into JSON (chat.scheduleMessage). Instead of continuing to apply these to each method individually as a patch, I thought it would be nice to solve this once and for all.

New groups of methods or new options that require encoding will require changes to accommodate those, but no patching. This is now being pulled from data in slack-api-ref and will update automatically with the docs.

I'm open to suggestions on the exact approach, and I'm opening this as a draft without tests, changelog, etc. to gather comments first.

[dblock]

I like this very much! See my suggestions/comments below. Definitely needs tests.

[jmanian]

@dblock Here's a different take, trying to base it entirely on data in slack-api-ref. It feels a little fragile to base it on data in the argument descriptions, but there isn't anything more structured at the moment (in slack-api-ref or on the doc pages). It does find all the ones we knew about, plus a handful of others.

[dblock]

This is a lot better! I think you should modify slack-api-ref to make it less fragile, aka parse the description for json in its scraper, and generate a field.

[jmanian]

@dblock still needs tests and changelog, but here's what this looks like now with the slack-ruby/slack-api-ref#64

[dblock]

Looks good!

[dblock]

Let's bump the version to 2.2 and make a release?

[jmanian]

Makes sense. I'm going to put up another PR to replace some of the patched argument validations with the new arg_groups data from the ref, so I'll do it in that.