Add public endpoint for contract change tariff wizard

CHANGELOG:

### Added
- [#641](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/641) Add new public API endpoint to call change tariff wizard to apply changes in existing mobile contracts 

Related card: https://trello.com/c/EgOqix0i/792-endpoint-de-canvi-de-tarifes-a-lapi-p%C3%BAblica-dodoo

---

> curl \
  -X POST \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Basic b2RvbzpQWEVyZVVnbzc3Y3E4M1J0QUEzR1MyTFRz' \
  --data '{
    "phone_number": "656565656",
    "product_code": "SE_SC_REC_MOBILE_T_UNL_2048"

  }' \
  "http://odoo-sc.local:8069/public-api/change-tariff"
> {"result": "OK"}

jobs:

Canvi de tarifa creat al contracte: