[vdk-core] Ingestion payload is serialized and validated before pre-handlers payload curation

[exalate-issue-sync]

Ingestion payload is serialized and validated immediately after payload submission, before being enqueued. Yet, the pre-processors (that are designed to be leveraged for payload curation when needed) are evaluated after data serialization&validation.

Either serialisation & validation needs to be evaluated after pre-processors have been invoked. Or at least validation needs to be optionally configurable (bool, eventually enabled by default) - so it could be at least moved(dupl) as a preprocessor if needed.

Find PR in progress [https://github.com//pull/777] to work on

Acceptance criteria: ingestion preprocessors are invoked before any payload serialisation & validation is performed