Add contract groupby fields

CHANGELOG:

### Added
- [#652](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/652) Add contract groupby fields

Related card: https://trello.com/c/x7Olqwcf/1004-agrupacions-per-defecte-a-les-vistes-tree-de-contracte

---