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

[XiaoYou201]

What type of PR is this：

• [x] 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]

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[XiaoYou201]

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