[UBP] Model free plan and other subscriptions

[svenefftinge]

The current free plan is 50h per month. We need to make sure that new users get a monthly credit entry for that.

Design Proposal

We will enhance the CostCenter type to contain information about a billingStrategy. There will initially be two strategies: stripe, other.

All cost centers with a stripe strategy are managed by stripe. So we will treat them as we do now (i.e. update the credits in every usage reconciliation and wait for Stripe to come up with an invoice finalization.

The other cost centers will be managed by a control loop, that inserts a credit into usage that is min(balance, usageLimit). This happens monthly.

[svenefftinge]

I'm adding a nextBillingTime field as well that the control loop can use to find cost centers that need to be billed.

[svenefftinge]

fixed by #13016