Allow to edit custom_name from a product attribute value

CHANGELOG:

### Fix
- [#722](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/722) Allow to edit custom_name from a product attribute value

Related card: https://trello.com/c/lqUzDW94/1133-els-productes-creats-per-la-ui-no-apareixen-a-formularis

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