feat(route53): Convenience API for creating zone delegations

[RomainMuller]

When creating delegation relationship between two PublicHostedZones,
one can now use zone.addDelegation(otherZone) instead of manually
creating the ZoneDelegationRecord insteance. This reduces the risk of
passing the incorrect name server, or hosting the record on the wrong
end of the relationship (DNS is hard!)

Additionally, fixes a bug in which it was not possible to create a zone
delegation record using a IHostedZone.hostedZoneNameServers property
as the array was mapped, which caused the Fn::GetAtt stringified list
token to become corrupted.

Fixes #1847

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.