Allow structured errors in rules

[jpreese]

This is based heavily off of the work done by @gwkunze in #100. Master has diverged quite a bit since the original discussions and PRs, so the important bits were cherrypicked from there.

This PR enables rego rules to use structured errors in their rules

e.g.

warn[msg] {
  kubernetes.is_service
  msg = sprintf("Found service %s but services are not allowed", [name])
}

could now be

violation[{"msg": msg, "details":{"additional": "extra stuff here if wanted"}}] {
  kubernetes.is_service
  msg = sprintf("Found service %s but services are not allowed", [name])
}

This will enable us to support Gatekeeper policies that require a rule header of

violation[{"msg": msg, "details":{}}]

[boranx]

@jpreese overall looks good
I'd prefer to merge it after commits are squashed into one.

[jpreese]

@boranx once approved we can click the Squash and merge button. I usually don't bother squashing until that point.

[boranx]

lgtm