Nested acorn validation support for acorn update --confirm-upgrade (#1726)

[keyallis]

#1726

Abstracted out validation support for nested acorn validation on app server endpoints.

Verified that acorn upgrade is still blocked from being called on nested acorns.
Verified that acorn upgrade --confirm-upgrade is allowed to call nested acorns.
Verified that the above call triggers an update to the nested acorn.

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• Commits follow contributing guidance
• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)

[thedadams]

Just a couple nits, otherwise looks good.

[ibuildthecloud]

acorn update --pull should work for nested acorns. Is that the case? All other updates should be blocked.

[g-linville]

acorn update --pull should work for nested acorns. Is that the case? All other updates should be blocked.

@ibuildthecloud I don't remember if --pull works but imo --confirm-upgrade needs to be allowed as well. Otherwise this configuration becomes useless:

acorns: "my-acorn": {
  image: "myimage"
  autoUpgrade: true
  notifyUpgrade: true
}