swh.webhooks.Webhooks: Add endpoints management methods

Define methods used to manage user endpoints receiving webhook messages.

The following operations are supported:

• creating a user endpoint

• retrieving endpoints associated to a specific event type

• retrieving the secret of an endpoint

• deleting an endpoint

A webhook endpoint is defined by an URL, an event type, an optional channel and an optional dictionary of string metadata.

Related to #3 (closed) Depends on !1 (merged)