Feat(ContractMobileTariffChangeWizard): add existing shared bond option

CHANGELOG:

### Added
- [#1074](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1074) Example text

Related card: https://trello.com/c/7QBxSJeA/1691-wizard-de-canvi-de-tarifa-m%C3%B2bil-canvis-per-permetre-gestionar-compartides

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