Expose contracts by API

CHANGELOG:

### Added
- [#788](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/788) Expose contracts by API with new endpoint `/api/contracts`

Related card: https://trello.com/c/pZs7avU0/1083-definir-la-tarifa-10gbep-si-un-provisi%C3%B3-t%C3%A9-la-propiet%C3%A0ria-amb-fibra-mm

---

curl \
  -X GET \
  -H  "accept: application/json" \
  -H  "api-key: $APIKEY" \
  "$BaseURL/api/contract?partner_vat=ES07650352T" | jq

[
  {
    "id": 712,
    "code": "569",
    "customer_firstname": "Dana",
    "customer_lastname": "Mar",
    "customer_ref": "1122",
    "customer_vat": "ES07650352T",
    "phone_number": "7897575744",
    "current_tariff_product": "SE_SC_REC_MOBILE_T_150_1024",
    "ticket_number": "3463626235",
    "technology": "Mobile",
    "supplier": "MásMóvil",
    "lang": "en_US",
    "iban": "ES6000491500051234567892",
    "is_terminated": false,
    "date_start": "2022-04-11",
    "date_end": false
  },
  {
    "id": 713,
    "code": "570",
    "customer_firstname": "Dana",
    "customer_lastname": "Mar",
    "customer_ref": "1122",
    "customer_vat": "ES07650352T",
    "phone_number": "6978979644",
    "current_tariff_product": "SE_SC_REC_MOBILE_T_UNL_10240",
    "ticket_number": "7889867676",
    "technology": "Mobile",
    "supplier": "MásMóvil",
    "lang": "en_US",
    "iban": "ES6000491500051234567892",
    "is_terminated": false,
    "date_start": "2022-04-11",
    "date_end": false
  }
]