Keap REST Hooks

Keap REST Hooks webhook surface. Subscribers register a `hookUrl` and `eventKey` via the v1 REST API (`POST /rest/v1/hooks`), complete an `X-Hook-Secret` verification handshake, then receive HTTP POST deliveries when contact, contactGroup, opportunity, invoice, order, and subscription events occur. Event types follow `noun.verb` dot-syntax (for example `contact.add`, `invoice.delete`).