feat(change_tariff): change tariff to sharing contracts

CHANGELOG:

### Added
- [#1082](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1082) When a new contract joins a shared bond, change tariffs according to the number of contracts sharing.

Related card: https://trello.com/c/RMDWjpN0/1733-ct-amb-abonament-compartit-si-passem-a-3-contractes-compartint-canviar-la-tarifa-dels-dos-primers

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