No way to test subscription webhooks in CLI or scenario simulator

[craigrileyuk]

Is your feature request related to a problem? Please describe.
At present, you can only test billing requests, mandates and payments for webhooks. Subscription functionality is heavily reliant on webhooks and so desperately needs support too.

Describe the solution you'd like
Addition of at least 'payment_created', 'cancelled', and 'finished' as supported events for either the GC CLI or the scenario simulator under the 'subscriptions' event type.