Resolve "Export via streaming serializer, introduce "Writer" abstraction"

This MR introduces streaming serializer, and refactor legacy json to use this streaming serializer.

• Introduces streaming serializer as a drop-in replacement for FastHashSerializer
• Introduces a Writer that can persist relations in different ways
• it would still produce a "fat JSON" so no ndjson here yet
• there would be no structural changes here yet, it's mostly a refactor

Closes #208803 (closed)