feat: unfold json with intermediate level

[adrienaury]

Schema definition

  - name: DATE
    length: 8
    schema:
      - name: SS
        length: 2
      - name: AAMMJJ
        length: 6

Should accept both JSON to unfold

{"DATE": "20250201"}

{"DATE": {"SS": "20", "AAMMJJ": "250201"}}