Registers a new subscriber endpoint to receive notifications. A subscriber represents an application or service that wishes to receive data change notifications for users who have granted consent. Endpoint Verification: For a subscriber to be successfully created, the provided endpoint_uri must be a valid HTTPS endpoint and must pass an automated verification check. The backend will send two HTTP POST requests to the endpoint_uri: 1. Verification with Authorization: * Headers: Includes Content-Type: application/json and Authorization (with the exact value from CreateSubscriberPayload.endpoint_authorization.secret). * Body: {"type": "verification"} * Expected Response: HTTP 201 Created. 2. Verification without Authorization: * Headers: Includes Content-Type: application/json. The Authorization header is OMITTED. * Body: {"type": "verification"} * Expected Response: HTTP 401 Unauthorized or 403 Forbidden. Both tests must pass for the subscriber creation to succeed. If verification fails, the operation will not be completed and an error will be returned. This process ensures the endpoint is reachable and correctly validates the Authorization header.