Add WebhookPayload type to aggregate all possible WebhookPayloadSchemas under one type union

[maxijonson]

This mapped type creates a union of all the WebhookPayloadSchemas definitions and does the following:

• Maps the event_type to the correct string literal associated with each event (rather than being a union of all possible event types)
• For event types that are listed in WebhookEventType but not in WebhookPayloadSchemas, the mapped type still acknoledges its existence but leaves the body as an unknown record.