public static byte[] getChannelId(SSLEngine engine) throws SSLException {
        return toConscrypt(engine).getChannelId();
    }
Gets the TLS Channel ID for the given server-side engine. Channel ID is only available
once the handshake completes.

@param engine the engine
@return channel ID or {@code null} if not available.
@throws IllegalStateException if this is a client engine or if the handshake has not yet
completed.
@throws SSLException if channel ID is available but could not be obtained.
common/src/main/java/org/conscrypt/Conscrypt.java