Support billingAddress pre-fill on POST /checkouts (like billing-update-link already does)

POST /subscriptions/{id}/billing-update-link already accepts a billingAddress object to pre-fill the hosted billing-update form ("Pre-fill billing address fields. Customer can modify these values in the hosted form."). POST /checkouts has no equivalent — customerId only pre-fills the customer's email, and PATCH /customers/{id} explicitly ignores address fields.

For merchants who already collect billing details in their own onboarding/account-settings UI, this means the customer has to re-enter the exact same address/company/tax info from scratch on the initial checkout page, even though Vatly already supports pre-filling it on a later billing-update flow.

Request: extend CreateCheckoutRequest with the same optional billingAddress pre-fill pattern already used on billing-update-link, so the initial hosted checkout page can start pre-filled too.