feat(clustering/sync): flatten and report validation error

[chobits]

Summary

1. flatten validation error for sync deltas
2. call RPC notify to API kong.sync.v2.notify_validation_error (spec: https://github.com/Kong/openrpc_specfiles/pull/18/)
3. fix bug that CP will crash if DP calls a non-existent RPC notify: KAG-6381

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-6351 KAG-5898 KAG-6381

[chronolaw]

We should open a same PR in CE.

[chobits]

We should open a same PR in CE.

this is a CE pr now

[chobits]

For pre-running ee test cases, see this pr: https://github.com/Kong/kong-ee/pull/11363

[chronolaw]

I think this PR is good enough, need another reviewer to confirm.

[lhanjian]

no e,

is latest_version

[StarlightIbuki]

Should we associate the error with a specific call?

[chobits]

yes we could. but it might be a TODO ticket later. Need to have an idea how to recover this with our product manager.

[chobits]

no e,

is latest_version

all fixed.

[chobits]

example that cp received from dp, the payload is as follows:

{
  error = {
    code = 21,
    fields = {},
    flattened_errors = { {
        entity_id = "8ef87a74-4e7f-4996-a92f-6540ee8d50bf",
        entity_name = "service-001",
        entity_type = "service",
        errors = { {
            field = "host",
            message = "required field missing",
            type = "field"
          } }
      } },
    message = "sync deltas is invalid: {}",
    name = "sync deltas parse failure",
    source = "kong.clustering.services.sync.validate.validate_deltas"
  },
  version = "v02_test_version"
}