Allow multiple events to be sent

[IanKemp]

Is your feature request related to a problem? Please describe.
In our live instance, GoCardless sends us single webhooks containing multiple events. For performance-testing purposes we'd like to simulate this behaviour in our development environment using the GC sandbox, but there is no way to do this; we cannot precisely control the event payload that the simulator sends to us.

Essentially we'd like to take a webhook that we received in our live instance and play it against our dev environment, but this is of course impossible due to the fact that the webhooks are signed. We do not want to disable signature verification in our development environment as this would introduce the risk of this code making it to production.

Describe the solution you'd like
An ability to submit any arbitrary JSON payload to the GoCardless sandbox, and have it forward that JSON to a specific endpoint listening to webhooks, with the correct signature in the headers.