[FEATURE] support explain statement

[arjenzhou]

Is your feature request related to a problem? Please describe.
when exectuing explain sql, throws java.sql.SQLException: create download session failed

comment in source:

* 在 Instance 上创建下载会话
   *
   * 非法情况:
   * 1. 非 SQlTask
   * 2. 非 select sql
   * 3. Task 非 Success 状态

Describe the solution you'd like
support explain query.

[xjr7670]

I had the same question. But it is not saying "create download session failed".
just a logview URL after the "Warning" label, and then "explain executed successfully". Could not see the explain statement's result. I must open the logview URL with browser to see the result