Core Enrollments Extensions

[pozylon]

• Allow to define the next end date of a subscription given a reference date and some meta data, allowing for complex terms regarding minimum contract duration, time conditions like prior note requirements of certain days. This should open up the system to handle termination events based on adapter logic so the end date of a subscription is completely up to the developer and rules he sets.
• Allow to define a subscription to automatically terminate at some point in time (no auto-renewal), supporting cases where you commit to 1 year but pay monthly or cases where you want to phase out existing plans.
• When an enrollment is initialized, an adapter should be able to calculate the state of "periods" initially, pre-generate multiple periods that go back in time or go far into the future. This allows to potentially adjust the start timestamp to the beginning of a month/week/day or even extend it to allow adding enrollments that have started a long time ago. / Currently it's only possible to calculate ONE period (the nextPeriod).