Make additional checkout fields readable and possible to target with an error

[ct-Marvin]

Shopify recently introduced additional checkout fields, as mentioned here. With checkout extensions, it is possible to split, for example, the address1 field and, by doing so, read the data entered into the additional field. However, this capability does not yet exist for cart and checkout validation functions. To perform precise validations and display relevant error messages, the additional fields should be considered in the Cart & Checkout Validation API.

Suggestion:

1.	Make the additional fields readable by adding them to the MailingAddress Object ([see here](https://shopify.dev/docs/api/functions/reference/cart-checkout-validation/graphql/common-objects/mailingaddress) )
2.	Add the additional fields as a target for displaying error messages. [see here](https://shopify.dev/docs/api/functions/reference/cart-checkout-validation/graphql#supported-checkout-field-targets)