GitOps driven Tofu plan, apply, iterate

To avoid having too many tools our developers need to focus on we would like to have a full GitOps driven Spacelift process. With merging of a PR when all changes are applied for the given stack(s) the PR refers to.

And being able to iterate on a PR to reach the desired end result before merging it.

There also needs to be a mechanism to avoid multiple PRs applying changes to the same stack at the same time, to avoid changes being inadvertently reverted.