DEV-135811: Support digital wallets in the SDK

[tomas-amaro]

Summary

Adds a digital-wallets payment-details model to the Java SDK, enabling merchants (driver: lastminute.com) to submit Apple Pay / Google Pay / Samsung Pay / WeChat Pay / Amazon Pay / Alipay payments. Implements the digital_wallets_payment_details schema from the API reference.

Jira: DEV-135811

Changes

• New model WalletPaymentDetails (com.riskified.models) implementing IPaymentDetails. Contains all 21 fields from the spec (payment_type, avs_result_code, credit_card_company, credit_card_country, credit_card_token, cardholder_name, authorization_id, mid, id, stored_payment_created_at, stored_payment_updated_at, installments, acquirer_bin, acquirer_region, authorization_type, expiry_month, expiry_year, initial_payment_amount, payment_frequency, billing_address_id, authentication_result).
• Registered the subtype in JSONFormater.paymentDetailsSerializer() (discriminator digital_wallet) so a wallet detail can be serialized inside an order's paymentDetails list. Reuses the existing AuthenticationResult and AuthorizationType models.
• Tests added to PaymentDetailsTest covering payment-type serialization, the net-new field keys, validation (happy path + rejections), and end-to-end polymorphic serialization.
• Version bump 6.3.1 → 6.4.0 in pom.xml and README.md.

Design decisions

• payment_type is a required, settable field (unlike other payment models which hardcode their type) since it varies across the six wallet types. It's a constructor argument and validated to be one of the six wallet values.
• Reused the existing PaymentType enum — all six wallet values already existed.
• initial_payment_amount, payment_frequency, billing_address_id are net-new to the codebase; optional numerics use boxed/nullable types so they only serialize when set.
• Required fields (payment_type, authorization_id, avs_result_code) match the spec.
• The method discriminator is a legacy SDK-internal field (not part of the API spec, which distinguishes purely by payment_type); registration is required by the polymorphic serializer, so digital_wallet was used for consistency with the other payment types.

Testing

• PaymentDetailsTest — all 15 tests pass (8 pre-existing + 7 new wallet tests), including verification that a wallet inside an IPaymentDetails list serializes with "method":"digital_wallet" and the correct payment_type.