swh.webhooks.Webhooks: Add method to send event to endpoints
Add a method to send an event of a given type with a JSON payload to registered endpoints.
The event can also be sent to a specific channel, only endpoints listening to this channel will receive it.
Before sending an event, the payload content is checked against the JSON schema of the event and an exception is raised if the validation fails.
Related to #4 (closed) Depends on !2 (merged)
Edited by Antoine Lambert