fix: Force flattened record according to provided flattened schema

[joaopamaral]

If a flattened schema is provided, and there is a dynamic object field, the flattened record can ignore the provided schema. This means that the resulting output may have more fields than what was originally specified in the provided schema. This can cause problems if the loader does not accept fields that are not listed in the schema.

So this PR is checking if a flattened is provided and forcing the flattened record to use it during the flatten record call.

Example:

RECORD:

{
    'key_1': 1,
    'key_2': {
        'key_3': 'value',
        'key_4': {
            'key_5': 1,
            'key_6': ['a', 'b']
        }
    }
}

SCHEMA:
{
    'key_1': {'type': ['null', 'integer']},
    'key_2__key_3': {'type': ['null', 'string']},
    'key_2__key_4': {'type': ['null', 'object']},
}

WRONG RESULT:
{
    'key_1': 1,
    'key_2__key_3': 'value',
    'key_2__key_4__key_5': 1,
    'key_2__key_4__key_6': '["a", "b"]'
}

EXPECTED RESULT:
{
    'key_1': 1, 
    'key_2__key_3': 'value', 
    'key_2__key_4': '{"key_5": 1, "key_6": ["a", "b"]}'
}

---

📚 Documentation preview 📚: https://meltano-sdk--2243.org.readthedocs.build/en/2243/

[joaopamaral]

added new test

[codspeed-hq]

CodSpeed Performance Report

Merging #2243 will not alter performance

_{⚠️ No base runs were found}

_{Falling back to comparing joaopamaral:fix/flatten_record_generating_non_expected_fields (938d15e) with main (e3565ee)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e3565ee) 88.54% compared to head (938d15e) 88.54%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2243   +/-   ##
=======================================
  Coverage   88.54%   88.54%           
=======================================
  Files          54       54           
  Lines        4724     4724           
  Branches      920      920           
=======================================
  Hits         4183     4183           
  Misses        380      380           
  Partials      161      161           

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[edgarrmondragon]

Thanks for contributing @joaopamaral! I left a few comments.

[edgarrmondragon]

Thanks @joaopamaral!