Change visibility of PaymentGatewayBase::getRemoteCustomerId() to public

Describe your bug or feature request.

For certain payment gateways, e.g. Authorize.Net, subsequent transactions with reusable payment methods require passing the customer's payment gateway ID alongside the payment method's ID. E.g. Authorize.Net subscription creation.

PaymentGatewayBase::getRemoteCustomerId() is available as a protected method within payment gateways but would require either extending a payment gateway or changing its plugin configuration to a class that provides a public method to use this logic. There is a legitimate use case for accessing this data from the payment gateway's PHP API. This data is not particularly sensitive, unique or opaque in the context of the gateway, since it is stored as a field on the user. (This field is added by commerce_payment as default installation config.)