Reactivate the subscription contract and optionally update the next billing date.

The subscription should be in paused or cancelled state and if the subscription is
cancelled, it cannot be a prepaid subscription. We will calculate a next billing date
based on the interval configuration if no nextBillingDate is provided.
A nextBillingDate without a time (YYYY-MM-DD) is a date in the shop time zone. It gets the shop renewal
billing time, or the time of the current next billing date when the shop has no renewal billing time.