add comment diff workflow

[zkamvar]

This workflow will run whenever one of use (the repo owners) adds the /diff comment in a pull request. It will generate a highlighted diff of the schema as a GitHub comment.

This uses two external actions

• r-lib/pr-fetch to check out the PR from a comment
• carpentries/comment-diff to create and subsequently update a comment with GitHub's API (I wrote this one at my previous job)