hook to amend params or override completeRedirectFlowWithGoCardless()

We working on a requirement to modify subscriptions in a way that for yearly memberships create direct debit with payment taken now and then next member payment taken Jan following year.

The solution we implementing is not very generic at this point, that we can submit to this extension, and providing a hook helps to keep the work clean and separate in another extension.

The pull request adds a hook around completeRedirectFlowWithGoCardless() which helps modify params or override the method, allowing further to modify the subscription schedule for specific cases.