TestWebhookInvokesFinalizeInvoiceRPC ensures that when the webhook is
hit with a `invoice.finalized` event, the `FinalizeInvoice` method on
the billing service is invoked with the invoice id from the event
payload.
Add a minimal Stripe event payload for an `invoice.finalized` event.
Ensure all tests send event payloads. The only event type of interest
for the webhook is `invoice.finalized` so other event types can have
empty payloads.
Test:
* The handler only accepts POST requests.
* The handler only responds to "invoice.finalized" events.
Add code to the handler to make those tests pass.
