[Feature] Post Changesets to PRs

[pbrisbin]

When someone opens a PR in the GitOps repo managed by CG, CG could run through every deploy it would do but with --no-execute-changeset and then post the JSON output of describe-changeset calls back to the originating PR. This would make the PRs incredibly reviewable, as you can see exactly the infra changes that would occur if merged.

I think this would require re-architecting the S3 side of things somehow. IIUC, there is presently a single bucket with a single state that triggers a deploy when changed. We would somehow need to synchronize the branch's version of things in a way that accurately triggers on what differs, but without actually persisting. If there's a less disruptively way to add this feature, do let me know.

This would also not work if you push a PR that has dependent changes across stacks -- though I don't think that's particularly well-supported even when actually executing the deploys today.