Fix/private products in catalog

⚠ Depends on !586 (merged) ⚠

---

CHANGELOG:

### Fixed
- [#615](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/615) Add private field to private products in the catalog.

Related card: https://trello.com/c/kNivFzzs/944-la-t-conserva-trenca-lapi-del-cat%C3%A0leg

---

With this flag, we avoid exposing these products in the catalog API.