This update enhances the system's ability to retrieve information related to signed documents. Previously, limitations prevented accessing additional data from the 'sign.send.request.signer' model when working with many-to-many signers. Now, customization is enabled, allowing for more comprehensive data retrieval, improving sign management workflows.

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