Comment only when costs change

[hugorut]

Currently we support

• Only post a comment in pull requests that have Terraform changes.
• Always post a comment in pull requests, regardless of file changes.
• Disabled

for Infracost comment logic (note this is limited to Github app/Gitlab app).

I've opened this issue to track adding support for "Only post a comment if costs increase". This would mean that Infracost only comments on PRs that increase the cost with the change. This could reduce noise and cognative load for certain users.