feature: support display execution plan

[QingZ11]

If a user executes an nGQL sentence using explain or profile, nebula should return the corresponding execution plan, which you can draw graphically in the studio, currently the console uses graphviz to generate the corresponding graph.

如果用户使用 explain 或者 profile 执行一句 nGQL 时，nebula 会返回对应的 execution plan，你们可以在 studio 中用图形的方式将这个 plan 画出来，目前 console 是通过 graphviz 来生成对应的图片。

ref：https://discuss.nebula-graph.com.cn/t/topic/2333/14