This update enhances the ability to retrieve information related to signed documents (o2m relationships) within the Odoo Enterprise system. Previously, limitations prevented accessing additional data from these relationships, now customization is enabled. This allows for more comprehensive tracking and reporting of signed transactions.

The class `SignerX2Many` is inheritable, but if we need another value from the o2m, we can't use it because the `signerX2Many` is not exportable.

Enable `signerX2Many` customization, allowing more fields to be read from the `sign.send.request.signer` model.

Forward-Port-Of: odoo/enterprise#99977