Integrate with Google pay / Apple pay

[omarlopesino]

Redsys allows integrating with both Goole Pay and Apple PAy:

https://pagosonline.redsys.es/googlePay.html
https://pagosonline.redsys.es/applePay.html

In short:

• To integrate with Google Pay it is possible to send 'google' in DS_MERCHANT_PAYMENT_METHODS
• To integrate with Apple Pay it is possible to send 'xpay in DS_MERCHANT_PAYMENT_METHODS. This method will also show Google Pay.

In the library, it is only needed to update the getAvailablePaymentMethods method so that these methods are available in dependent libraries (for example, commerce_sermepa module).

[omarlopesino]

Please review this MR that adds the feature #21 , thanks!