GitHub Action that translates spotless apply diff into GitHub pull request code suggestion comments

### Discussed in https://github.com/diffplug/spotless/discussions/1700

Spotless GitHub Action developed & applied which takes the diff as created through spotless:apply and uses for instance [googleapis/code-suggester](https://github.com/googleapis/code-suggester#Action) to create [code suggestion comments](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/commenting-on-a-pull-request) on the pull request.

This is similar to

- #655

but that issue is blocked on a few internal enhancements to Spotless. This issue uses `spotlessApply` to change the files in the working copy, and then generate PR change suggestions from those changes in the working copy. That simplifies the implementation quite a bit, and should be workable with even very old versions of Spotless.

Happy to take a PR for this.