Allow recurring payments to override a payment method's gateway's transaction settings

Problem/Motivation

Subscriptions reference stored payment methods that will be used to process their recurring payments. These payment methods each reference a payment gateway whose configuration, including API credentials and transaction type, are used to submit the recurring payment. This means if a site wants only to authorize a payment during checkout, its recurring payments will similarly be authorization only. However, it's possible that a site may want only to authorize the initial checkout (i.e. in the case of a site that has partial fulfillment at first) but authorize and capture the recurring charges (i.e. in the case of a recurring fee processed after a magazine has already been mailed).

Proposed resolution

I don't know the best place to seat such a configuration option, but I believe we should provide an option somewhere for the subscription to indicate whether or not it recurring payments should use the transaction type specified by the related payment gateway or override it to use a fixed value (i.e. authorize and capture). We then would need to figure out how to supply that alternate transaction type to the payment gateway module, since it's responsible for processing its own payments, and I don't think we have a place currently when invoking the payment routines to override the gateway configuration.