swh.webhooks.Webhooks: Add method to list events sent to an endpoint
That new method returns a list of events sent to a given endpoint, with optional date filtering.
The info available for each sent event are the following:
- event type
- endpoint URL
- optional endpoint channel
- HTTP headers sent with the event
- JSON payload
- the date the event was sent
- the endpoint response
- the reponse status code
Related to #5 (closed) Depends on !3 (merged)