helper script to diff generated deploy artifacts

[joshk0]

Moved from pre-OSS repository

Sort of like a terraform plan, it would be really helpful to have a kdt helper script to diff a render_deploys output directory with the contents of a pre-existing build.

If this exists, you could go even further, have it use a webhook, and make it auto post the deploy diffs to any PR where kubernetes files get changed. This is optional for this story.

Acceptance criteria

• Tool exists which downloads a deploy artifact and diffs your current build directory against it, or, allows you to diff against a different official build.
• Documentation is updated