Add workflow for automatic API updates

[duffn]

This adds a workflow that automatically updates the API from slack-api-ref and creates a running PR with changes per #456.

Notes

• the repo will need to be updated to allow Actions to create pull requests in settings

• this setup section is silly but if there’s no PR, need to get the PR number in order to include it in the changelog: master...duffn:slack-ruby-client:duffn/automate-api-updates?expand=1#diff-75bb0e4f3b
• runs 1 hour after the slack-api-ref cronjob
• will either create or update an already existing PR from the automated-api-update branch

Example

This works here, but it wouldn't surprise me if this didn't quite work as expected after merging here, but can fix forward.

https://github.com/duffn/slack-ruby-client-dev/pull/5

Closes #456

[dblock]

Let's merge and see how it goes!

When there are no changes we get a PR. I think that's the next thing to fix.

[duffn]

Ah yeah, that needs changing. Will see how/if it runs later today and then address any necessary fixes.