fix(clustering/sync): insert default values to delta entities before validation

[chobits]

Summary

Try to align with sync v1 logic, v1 will ignore required fields error because

dc:validate(input)
....
  -> field_schema:process_auto_fields(value, "insert")
  -> route:validate

So when recursively calling the validate() function, it will call process_auto_fields before validating a specific entity inside for sync v1.

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 KAG-6249