Consider deprecating old middleware API

**Description**
Currently parsers lacks proper serialization support.

**Proposed solution**
Add configurable serialization with sensible defaults.

**Priority**: Low/Medium

Start by locating the parser and middleware API entry points referenced in the issue, then trace how parser output is currently handled. Define the configurable serialization behavior and sensible defaults before changing anything; done means the old middleware API decision and serialization behavior are documented and covered by relevant tests.

Written by the indexing model from the issue text.