@Nullable
    default TransportType transportType() {
        return null;
    }
Returns the {@link TransportType} that will be used for socket I/O in Armeria.

<p>The default value of this flag is {@code "epoll"} in Linux and {@code "nio"} for other operations
systems. Specify the {@code -Dcom.linecorp.armeria.transportType=<nio|epoll|io_uring>} JVM option to
override the default.</p>
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java