Added support for OpenPath gateway

Spreedly now supports the OpenPath gateway. Merchants can add OpenPath as a gateway and process transactions through gateway_type: openpath. Supported operations include purchase, authorize, capture, partial capture, refund, partial refund, void, verify, store, and unstore. 3D Secure is supported through Spreedly 3DS2 Global flows. Stored credentials are supported for recurring, installment, and unscheduled transactions. Store returns OpenPath-vaulted third_party_token payment methods for follow-on transactions. Apple Pay, Google Pay, and Level 2/3 data are not currently supported for OpenPath. For further information, see the OpenPath gateway guide.