[#369][Bugfix] Change the starrocks connector UNKNOWN data type handle method

[XiaoYou201]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

Fixes #369

Problem Summary(Required) ：

The starrocks connector handle UNKNOWN data type as same as json. It is unreasonable. So, in my opinion ,it need to be modified.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

vinnerzhang seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[XiaoYou201]

what's DCO? I'm not clearly about the pr flow. Could anyone give me some suggestion? thx~