When calculating the unknown values for JSON plan output, we would
previously recursively call the `unknownAsBool` function on the current
sub-tree twice, if any values were unknown. This was wasteful, but not
noticeable for normal Terraform resource shapes.

However for deeper nested object values, such as Kubernetes manifests,
this was a severe performance problem, causing `terraform show -json` to
take several hours to render a plan.

This commit reuses the already calculated unknown value for the subtree,
and adds benchmark coverage to demonstrate the improvement.