Magento 2 Add Extra Address Fields to Checkout

• It renames extra address attribute fieldname, datascope to customAttributes.{attribute_code} LayoutProcessor
• With a javascript mixin it transports them from customAttributes to extension_attributes
• Plugin
• In a event is transport the values from object to object. Example Quote Address to Order Address

Usage:

1. Add a fieldset.xml

   <scope id="global">
       <fieldset id="extra_checkout_billing_address_fields">
           <field name="attribute_code">
               <aspect name="to_order_address" />
               <aspect name="to_customer_address" />
           </field>
       </fieldset>
   </scope>

2. Add a extensions_attributes.xml
3. Add a customer_address attribute setup
4. Add a quote_address field setup
5. Add a order_address field setup

Example: See the module ExtraCheckoutAddressFieldsTest