Registers a URL of your system to receive messages of the types in event_types. The response carries the secret for checking the signature of every message. It is shown only in this response, so store it now; if you lose it, rotate the signing secret. The URL must be https, without user:pass@, and every IP its domain resolves to must be public (one internal address and we refuse); this is checked again on every send. An account can have up to 16 endpoints. The account owner gets an email each time an endpoint is added. Needs an API key with webhook:manage.
