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)