Integrate with Sylius/RefundPlugin for partial refunds

[loevgaard]

Answers the oldest ask in this tracker — #27 (2021) requested partial refunds — and the timing is finally right: payum-quickpay gained refund_amount/capture_amount support in 2.0.0-alpha.2.

The integration contract (standardized across gateway plugins)

Verified against RefundPlugin source and four real integrations (Sylius/AdyenPlugin, Sylius/MolliePlugin, payplug/SyliusPayPlugPlugin, hipay/hipay-payments-sylius):

1. Whitelist the gateway: add quickpay to the sylius_refund.supported_gateways container parameter (defaults to offline only) — RefundPlugin's SupportedRefundPaymentMethodsProvider filters by gateway factory name, so without this the refund UI never offers Quickpay payment methods.
2. Handle the event: a Messenger handler on Sylius\RefundPlugin\Event\RefundPaymentGenerated (carries the refunded amount, currencyCode, and the related paymentId) that guards for Quickpay methods and executes the gateway refund with that amount.
3. Complete the RefundPayment: apply complete on the sylius_refund_refund_payment graph via RefundPaymentCompletedStateApplier. Quickpay refund operations are confirmed by callback, so completion should follow the Adyen model (complete when the gateway confirms) rather than Mollie's immediate auto-complete.

Details worth copying

• Adyen's manual-complete guard: FilterManualRefundConfirmationSubscriber rejects the manual sylius_refund_refund_payment.complete transition for its methods, so only a gateway-confirmed refund completes — cheap correctness guarantee.
• Mollie's optionality: sylius/refund-plugin in suggest (not require) with the whole integration behind a conditional config import (config/integration/refund-plugin/), so shops without RefundPlugin are unaffected.

Tasks

• Optional-dependency wiring (suggest + conditional service registration when RefundPlugin is installed)
• supported_gateways parameter, RefundPaymentGenerated handler, callback-driven completion
• Reconcile with the existing full-refund path in PaymentProcessor (avoid double refunds when both mechanisms are active)
• README section + note in UPGRADE-2.0.md; closes Partial refund (and capture) #27's ask on the refund half