Add /api/fiber-contracts-to-pack endpoint

Returns available fiber contracts to pack with from a given user. Add fiber_signal_type parameter.

CHANGELOG:

### Added
- [#909](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/909) Add fiber-contracts-to-pack API endpoint

Related card: https://trello.com/c/WQnEnvGT/1438-api-contracts-retorna-el-contractes-banda-ampla-fibra-no-associats-a-un-m%C3%B2bil-no-pack

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