Configures the outbound event webhook. The endpoint URL is validated (must be https and must not resolve to an internal/private/metadata address). Partial-update semantics: omit customHeaderValue to keep the stored secret, omit customHeaderName to keep the stored name, and omit the events object — or any individual event key — to keep the stored subscriptions; only keys you send are applied. The events.* keys use the getWebhookEventTypes vocabulary (emailReported, payloadClicked, deliveryFailed, trainingFailed) so they match getWebhook's response 1:1.