Add CRD Upgrade Safety preflight check

[everettraven]

What this PR does / why we need it:

Adds an initial implementation of carvel-dev/carvel#729

Includes the new CRDUpgradeSafety preflight check implementation and 2 of the known breaking changes during a CRD upgrade:

• Scope change (i.e cluster <--> namespace)
• Removal of existing stored version

Implementation is broken down into individual commits for review purposes, I am happy to squash commits before merging:

• Initial, generic preflight check implementation: c2a34a5
• Scope change validation: 228ead6
• Stored version validation: 28d6094

Which issue(s) this PR fixes:

Fixes #909

Does this PR introduce a user-facing change?

Adds an initial `CRDUpgradeSafety` preflight check implementation with checks to ensure a CRD upgrade does not include a scope change and/or removal of existing stored versions

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'm planning to squash commits prior to merging, but after reviews have been done to keep it easier to review

[everettraven]

Note: I'm planning to squash commits prior to merging, but after reviews have been done to keep it easier to review

Whelp a botched rebase caused some problems that ended up in me deciding a squash after the rebase would be cleaner than the individual commits. Apologies to the reviewers!

[praveenrewar]

LGTM! Thank you so much for making kapp better ❤️
I think a rebase would be required as Todd's PR is merged.

[everettraven]

Thanks @praveenrewar ! I've just rebased, updated, and squashed the commits. Should be ready to merge!

The update made was https://github.com/carvel-dev/kapp/pull/901/files#diff-02d1be65fcbbdbd73025e5327506d584006201a288ba1c598b92e7d67467529eR52-R54 to satisfy the change to the preflight.Check interface Todd had made.