Add handling of updates to a field's enum values to the CRD Upgrade Safety preflight check

[everettraven]

What this PR does / why we need it:

• Adds a generic, extensible validator for handling changes to existing fields in an existing version of a CRD schema
• Adds an initial field change validator for handling changes to an existing fields allowed enum values
  • Rejects when the existing version of the field does not have enum restrictions but the new version adds enum restriction
  • Rejects when enums from the existing version of the field are removed in the new version
  • Approves when enum values are added to the list of allowed enum values

Which issue(s) this PR fixes:

Fixes #916

Does this PR introduce a user-facing change?

Add logic to the CRD Upgrade Safety pre-flight check to only allow additions to the valid enum values of an existing field in an existing version of a CRD schema

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.:

[everettraven]

Note: I'll squash commits after reviews

[praveenrewar]

Looks pretty good apart from some minor comments.

I also wanted to mention one additional thing apart from the comments. In the Carvel world, we have been capitalising error strings (examples can be found across the projects). I know, I know, this is not as per the go style decisions, however since this is being done across the carvel tool suite, the error messages are consistent within Carvel. This might be something worth discussing outside the scope of this PR (maybe in the community meeting?). At this point I don't want to force you to capitalise error strings as even now when we get errors from some other library, they are not capitalised and the error messages are a mix of the two, which would be something similar with preflight checks.

[rashmigottipati]

Changes look good to me!

[praveenrewar]

Looks good!!