Feat(CoopAgreement): add default code value when duplicating model

CHANGELOG:

### Changed
- [#1073](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1073) Allow to duplicate Coop Agreement instances keeping the unique code constraint by adding a new default code when copying the model

Related card: https://trello.com/c/DHSSoSM1/1705-volem-poder-duplicar-convenis-cooperatius-per-la-ui

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