static int byteAtIndex(int mask, int index) {
        return (mask >> 8 * (3 - index)) & 0xFF;
    }
Generates Sec-WebSocket-Accept using Sec-WebSocket-Key.

@see <a href="https://datatracker.ietf.org/doc/html/rfc6455#section-1.3">Opening Handshake</a>
core/src/main/java/com/linecorp/armeria/internal/common/websocket/WebSocketUtil.java