A callback to indicate that a client connects to receive Server-Sent Events. The application has full access to HTTP request and can decide whether to accept the connection or reject it. In SSE, clients will reconnect if the connection is closed, but can be told to stop reconnecting by returning the appropriate status.
Last-Event-ID may be used in determining the status and it can be got using {@code HttpServletRequest.getHeader("Last-Event-ID")}
@param request connection request
@return Status to accept, or don't reconnect etc