feat(ca_certificates): make ca_certificates cert referenceable

[windmgc]

Summary

This PR makes the cert field of the ca_certificates entity referenceable.

The ca_certificates entity has a cert_digest field which is calculated by transformation on_write functions. For referenced CA certs we cannot calculate the cert_digest when writing the entity to the database, because the entity creation always happens on CP and CP may not be able to dereference a secret. Therefore, the PR changed the behavior that if a referenced cert value is being used to create a ca_certificate entity, the cert digest must be manually provided too, otherwise it will not pass the entity check.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix https://konghq.atlassian.net/browse/FTI-6231