Added support for PayPlus gateway

Spreedly now supports the PayPlus gateway. Merchants can add PayPlus as a gateway and process transactions through gateway_type: payplus. Supported operations include purchase, authorize, capture, refund, partial refund, void, verify, general credit, store, and unstore. 3D Secure is supported through Spreedly 3DS2 Global flows. Apple Pay and Google Pay are supported for purchase, authorize, refund, and void. Store returns PayPlus-vaulted third_party_token payment methods for follow-on transactions. Installments are supported through the installments and credit_terms gateway-specific fields, including issuer-financed credit installments (credit_terms: 6) and automatic order-value tiers for ILS transactions; installment details are returned in gateway-specific response fields. Transaction retry / decline salvage is supported. PayPlus is available in Composer. The order_id is sent to PayPlus for reporting. Partial capture and gateway-specific 3DS2 are not currently supported for PayPlus. For more information, see the PayPlus gateway guide.