Base for Electronic Tax Document for Chile

This module is a technical foundation for Chilean electronic tax documents (DTE). It does not add a new menu by itself; instead it prepares the connection to the Chilean tax authority (SII) and adds rules used by other Chilean localization modules.

What it changes: - It creates a preconfigured connection record named "Servicio de Impuestos Internos (SII) de Chile" pointing to sii.cl, and links Chile as the country using that connection. - On the shared electronic tax document mixin (etd.mixin), it adds a field called "SII Document" (class_id) that links each document to a type of SII document. This field is tracked, so changes are recorded in the document history. - On companies (res.company), it adds a check: if a company selects the SII connection as its signing provider, Odoo raises an error saying that SII does not sign documents and asks the user to choose another provider.

Because it depends on connector, document, folio, activity, reference and partner email modules, it is meant to be installed together with the rest of the Chilean electronic invoicing suite. The module is marked as Beta.