@Nullable
    @UnstableApi
    default TlsEngineType tlsEngineType() {
        return null;
    }
Returns the {@link TlsEngineType} that will be used for processing TLS connections.

<p>The default value of this flag is "openssl", which means the {@link TlsEngineType#OPENSSL} will
be used. Specify the {@code -Dcom.linecorp.armeria.tlsEngineType=<jdk|openssl>} JVM option to override
the default.</p>
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java