Sponsees endpoint

CHANGELOG:

### Added
- [#818](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/818) Create new '/api/partner/sponsees' endpoint to get sponsees information from a given partner

Related card: https://trello.com/c/uqui4WSt/1247-nou-endpoint-per-apadrinaments

---

{
  "sponsorship_code": "epY2a",
  "sponsees_max": 5,
  "sponsees_number": 2,
  "sponsees": [
    "Lladruc Test",
    "Joana Test"
  ]
}

Or if not found:

{
  "code": 404,
  "name": "Not Found"
}

Deploy with https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio-python-client/-/merge_requests/35