VIES VAT Registry Validation

The registry half of EU VAT number validation as a free open-source gem: a validator that asks VIES whether a number is actually registered, records audit-grade evidence (the consultation number a qualified check returns), and keeps verdicts fresh over time. Core 6.0 ships only the offline half (format and checksum) with a contract that an extension re-registers the eu_vat kind and takes over both halves; this gem is that extension. The existing pipeline (statuses, async job, subscriber, admin validate endpoint) starts doing real work the moment the gem registers.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-vies-vat-validation.md

Tracked in Linear as V-3619.