feat(json-schema): support x-additionalPropertiesName

[eltoder]

x-additionalPropertiesName is a Redocly extension to display more descriptive property names in objects with additionalProperties[1]. This allows generating nicer looking and more informative documentation and examples than Swagger UI's current "additionalProp" name.

[1] https://redocly.com/docs/api-reference-docs/specification-extensions/x-additional-properties-name/

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• [x] Features (non-breaking change which adds functionality)

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