Register a webhook endpoint

Needs reservations:read as well as webhooks:manage. The endpoint you register receives whole reservations, so pointing one at a URL costs the scope that reads them. Registers an HTTPS endpoint to receive reservation events. The URL must resolve to a public address; we check that at registration. The response is the only time the secret is shown in full. Store it before you move on: every later read masks it, and the only way to get a usable secret again is to rotate it. An organization may hold at most 10 endpoints, and deactivated endpoints still count against that limit because endpoints cannot be deleted. To move traffic to a new URL once you are at the limit, point an existing endpoint at it with PUT. Unlike creating a reservation, this endpoint ignores the Idempotency-Key header, so a create you retry after a lost response can register a second endpoint that then receives every event alongside the first. Duplicates show up in GET /rs/v1/webhooks and can be switched off with PUT.