[ENH] Migrate OpenAI's json_struct mode to use prompt template

[paxcema]

Description

Deprecate input_text keyword argument in favor of prompt_template when using json_struct mode, both for UX streamlining and also to enable multi-column support.

Example queries change from this:

CREATE MODEL mindsdb.nlp_model
PREDICT json
USING
    engine = 'openai_engine',
    json_struct = {
        'rental_price': 'rental price',
        'location': 'location',
        'nob': 'number of bathrooms'
    },
    input_text = 'sentence';

SELECT json
FROM mindsdb.nlp_model
WHERE sentence = 'Amazing 3 bedroom apartment located at the heart of Manhattan, has one full bathrooms and one toilet room for just 3000 a month.';

To this:

CREATE MODEL mindsdb.nlp_model
PREDICT json
USING
    engine = 'openai_engine',
    json_struct = {
        'rental_price': 'rental price',
        'location': 'location',
        'nob': 'number of bathrooms'
    },
    prompt_template = '{{sentence}}';

SELECT json
FROM mindsdb.nlp_model
WHERE sentence = 'Amazing 3 bedroom apartment located at the heart of Manhattan, has one full bathrooms and one toilet room for just 3000 a month.';

Type of change

• ⚡ New feature (non-breaking change which adds functionality)
• 📢 Breaking change (fix or feature that would cause existing functionality not to work as expected)
• 📄 This change requires a documentation update

@martyna-mindsdb feel free to directly change docs on this PR if that's helpful.

Tagging @torrmal for visibility at his request.

Verification Process

To ensure the changes are working as expected:

• Test Location: manual test.
• Verification Steps: follow this example from our docs.

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.

[martyna-mindsdb]

@paxcema
Thanks. I have pushed the docs updates.