[ENHANCEMENT] argilla: simplify structure for flatten records to list

[frascuchon]

Pull Request Template

This PR changes the structure generated by to_list(flatten=True) to simplify reading responses. The response content is split into values and users, so no user ID is defined as part of the column name:

The result for the following record:

record = rg.Record(
    fields={"field": "The field"},
    metadata={"key": "value"},
    responses=[
        rg.Response(question_name="q1", value="value", user_id=user_a),
        rg.Response(question_name="q2", value="value", user_id=user_a),
        rg.Response(question_name="q2", value="value", user_id=user_b),
        rg.Response(question_name="q1", value="value", user_id=user_c),
    ],
    suggestions=[
        rg.Suggestion(question_name="q1", value="value", score=0.1, agent="test"),
        rg.Suggestion(question_name="q2", value="value", score=0.9),
    ],
)

is :

{
    "id": <record_id>,
    "_server_id": None,
    "field": "The field",
    "key": "value",
    "q1.responses": ["value", "value"],
    "q1.responses.users": [str(user_a), str(user_c)],
    "q2.responses": ["value", "value"],
    "q2.responses.users": [str(user_a), str(user_b)],
    "q1.suggestion": "value",
    "q1.suggestion.score": 0.1,
    "q1.suggestion.agent": "test",
    "q2.suggestion": "value",
    "q2.suggestion.score": 0.9,
    "q2.suggestion.agent": None,
}

Refs #4936

Type of change

• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

Checklist

• I added relevant documentation
• follows the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[burtenshaw]

This looks like a real improvement.

One thing though, because responses are an array of values, the flatten param is no longer true. i.e. the structure is not flat. Should we consider changing the parameter name? For example, we could do nested and default to True.