Substitute sponsor_id by sponsor_ref in partner API result

CHANGELOG:

### Changed
- [#872](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/872) Substitute sponsor_id by sponsor_ref in partner API result

Related card: https://trello.com/c/dAk5o4U5/1349-devolver-el-ref-del-sponsor-en-la-api-de-partner-en-lugar-de-sponsor-id

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