encrypting transformer for storing secrets encrypted at rest ([#41939](https://github.com/kubernetes/kubernetes/pull/41939), [@smarterclayton](https://github.com/smarterclayton))

  * Add secretbox and AES-CBC encryption modes to at rest encryption. AES-CBC is considered superior to AES-GCM because it is resistant to nonce-reuse attacks, and secretbox uses Poly1305 and XSalsa20. ([#46916](https://github.com/kubernetes/kubernetes/pull/46916), [@smarterclayton](https://github.com/smarterclayton))...