Webhooks: Fetch event w/ payload and headers

It could be useful to be able to get the event payload and request headers for a delivery.

One way to do this might be to add webhook_event_fetch endpoint. Since we include the event_id but that doesn't correspond with anything the user can access. It could include the event payload, headers and list of deliveries.

This would also give us the ability to show other deliveries that correspond to the same event. For example if you have a bunch of resends, we could list them all in one place.

Alternatively we flip it and replace the webhook_delivery_list with a webhook_event_list – which includes a list of events, each containing a list of corresponding deliveries.

Note that I'm coming at this from a UX point of view, so take with a pinch of salt.

From current webhook MVP designs: