Diff comment 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)

I've updated the README to highlight the automatic generation of diffs, while leaving @annakrystalli instructions for manually creating the diffs as a backup

[annakrystalli]

This looks great! Thanks @zkamvar

I'm going to wait till the review (which has already begun in #88) is complete and will merge this in after before merging into main.