Add field current_tariff_start_date in contract

CHANGELOG:

### ADDED
- [#879](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/879) Add field current_tariff_start_date in contract

Related card: https://trello.com/c/KAJekPzR/1381-afegir-camp-filtrable-data-dinci-de-la-tarifa-actual

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