Expose sponsorship code and sponsees limit

CHANGELOG:

### ADDED
- [#786](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/786) Expose sponsorship code and sponsees limit

Related card: https://trello.com/c/QfJU1Qoo/1235-exponer-c%C3%B3digo-apadrinamiento-api

---