Add a check for removing an existing field

[everettraven]

This builds on top of #661 and focuses on adding a check to ensure that performing an update operation to a CustomResourceDefinition does not result in an existing field being removed.

If the proposal for adding this functionality to carvel-dev/kapp has been accepted prior to starting this work, all changes should be made against carvel-dev/kapp, otherwise the changes should be made against https://github.com/everettraven/kapp/tree/feature/crd-upgrade-safety-preflight

[everettraven]

The proposal has been merged and a new issue has been created in carvel-dev/kapp to track this change. In turn, this is now a placeholder/tracker issue for carvel-dev/kapp#912

[rashmigottipati]

Closing as completed with the PR merged into carvel-dev/kapp: carvel-dev/kapp#922