Stub out the PaymentGateway plugin

The starting point for the integration should be stubbing out the PaymentGateway plugin. I believe the nearest equivalent will be the Braintree integration, given its Hosted Fields implementation is similar in kind to Adyen custom card integrations.

The basic settings the gateway will need to incorporate include the Merchant account, API key, and Client key (used for the card component). We'll also need options to enable 3D Secure and to select the live environment, as there's a single test environment but multiple live options as shown in step 2 of the Web integration.

When you look at the HostedFields plugin in the Braintree module, you'll see that we made use of a third party PHP library to instantiate an api property during gateway construction. We should do the same in the CustomCard plugin using the official PHP library.