Improve contract api with shared bond
CHANGELOG:
### Added
- [#1043](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1043) Improve the contract API with shared bond attributes and logic.
Related card: https://trello.com/c/w94x7DAl/1660-odoo-api-contracts-v2
Edited by Daniel Palomar