Enum for "format" prop in `DurationDatatypeType`

cf.

https://github.com/InseeFr/Pogues-Model/blob/a5046ef7641d726e6ae0d6052c8473cba5fb8480/src/main/resources/xsd/Questionnaire.xsd#L936

This format attribute has some specific valid values in Pogues use-case:

- `"PnYnM"` for a year/month duration
- `"PTnHnM"` for a hour/minute duration

An enum would be preferrable.

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Note (for the record): this comes from some old xml standard: https://datypic.com/sc/xsd/t-xsd_duration.html

An other option would be to implement validation methods for this prop. But I think that would be overkill four our use-cases.