public WebSocketWriter outbound() {
        final WebSocketWriter writer = WebSocket.streaming();
        setOutbound(writer);
        return writer;
    }
Returns the {@link WebSocketWriter} that is used to send WebSocket frames to the server.

@throws IllegalStateException if this method or {@link #setOutbound(Publisher)} has been called already.
core/src/main/java/com/linecorp/armeria/client/websocket/WebSocketSession.java