[EPIC] Add contract groups

CHANGELOG:

### Added
- [#1112](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1112) Add contract groups management.

Related card: https://trello.com/c/kUJubktF/1762-agrupaci%C3%B3-de-contractes

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• Add the contract group concept