Support new shopify-* webhook header format

Update Webhooks::Request to accept both the new `shopify-*` header
format (e.g., `shopify-hmac-sha256`) and the legacy `x-shopify-*`
format, with the new format taking precedence when both are present.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>