MX: do not validate presence of extensions for foreign customers

[samlown]

• I'd like to be able to define a customer as a foreign entity and not have GOBL raise an error due to missing Mexican fields.
• The options here will need to be validated with the gobl.cfdi package.
• Mexico differentiates between local and foreign B2C customers (XAXX010101000 vs XEXX010101000), so it should still be possible to add a client with just the tax_id country.