@Override
    public void writeExternal(ObjectOutput out) throws IOException {
        byte[] bytes = getBytes();
        out.writeInt(bytes.length);
        out.writeInt(this.numFields);
        out.write(bytes);
    }
Write the bytes of var-length field into ByteBuffer

Note: only work with HeapByteBuffer
indexr-query-opt/src/main/java/io/indexr/query/row/UnsafeRow.java