Add smart button to show related contracts in packs

CHANGELOG:

### Added
- [#902](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/902) Show pack contracts in smart button in Contract form

Related card: https://trello.com/c/gwef491y/1447-smartbutton-a-partner-per-visualitzar-tots-els-contractes-del-pack

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