Add a check for ensuring that existing required fields can be marked as optional

[rashmigottipati]

This builds on top of #666 and focuses on adding a valid check to ensure that existing required fields can be marked as optional in a CustomResourceDefinition schema.

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]

Proposal merged and work is now being tracked in carvel-dev/kapp. This is now a tracker for carvel-dev/kapp#915

[rashmigottipati]

Created a PR (in code review) for this in kapp: carvel-dev/kapp#933

[rashmigottipati]

Closing as PR merged in kapp upstream: carvel-dev/kapp#933