Allow EuropeanUnionVat to use a custom tax_number field

There are two different issues for adding tax numbers:
#2874149: Create a tax_number field type and use it on customer profiles
#2930722: Tax number validation and validation for tax number in the EU

As well as a contrib module: https://www.drupal.org/project/vat_number

Until we solve that, it would be good to have an easy way to override which tax number field is used by the EuropeanUnionVat plugin, so that we don't need to override the entire method. Perhaps via a constant?

Current code:

    // @todo Replace with $customer_profile->get('tax_number')->value
    // once tax numbers are implemented.
    $customer_tax_number = '';