Add validation to CRDUpgradeSafety preflight check to prevent removal of existing fields

[rashmigottipati]

What this PR does / why we need it:

Since the base CRDUpgradeSafety preflight check is in place and merged into the develop branch, we are at a point where we can continue adding validation logic based on the CRDUpgradeSafety preflight check proposal.
The validation logic in this PR focuses on adding a check using the CRD schema checker to ensure that no existing fields are removed from a particular version of a CRD's schema during an upgrade operation.

Which issue(s) this PR fixes:

Fixes #912

Does this PR introduce a user-facing change?

Add validation to `CRDUpgradeSafety` preflight check to prevent removal of existing fields from a particular version of a CRD schema during an upgrade operation

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[rashmigottipati]

cc @everettraven

[everettraven]

LGTM!

[praveenrewar]

LGTM! Thank you for your contribution 🙏🏻
I added some comments on Bryce's PR that maybe relevant to this as well (package name for test files and capitalising error messages), although no action item needed as of now.