Add sponsorship_hash to coop_candidate partners

CHANGELOG:

### Fixed
- [#795](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/795) Add sponsorship_hash logic to coop_candidates

Related card: https://trello.com/c/3w06YJRW/1256-candidat-a-s%C3%B2cia-no-esta-generant-codic-per-apadrinament

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In SC the coop_candidates and the members have the same privileges and in this case, both can sponsor.