why not use reflect to get InternalEngine and IndexWriter?

[zhangdapao745]

if use reflect to get InternalEngine and IndexWriter, we need not modify elasticsearch source code

[zhangdapao745]

AppendLuceneTransportAction::doExecuteCore

            /* FIXME 这里需要修改es的代码, 将lucene的IndexWriter对象暴露给plugin使用  */
            InternalEngine engine = (InternalEngine) shard.getEngineOrNull();
            IndexWriter indexWriter = engine.getIndexWriter();```