feat: Variable assignment supports type conversion and can be set to null

[wangliang181230]

What this PR does / why we need it?

Summary of your change

feat: Variable assignment supports type conversion and can be set to null
新特性：变量赋值 支持类型转换，以及可赋值为 null

Please indicate you've done the following:

1. 当为 自定义string 或 引用变量 时，允许选择转换类型，支持转换为：
   • string: 使用 str(val) 进行转换；
   • int: 使用 int(val) 进行转换
   • float: 使用 float(val) 进行转换
   • json_object: 使用 json.loads(val) 进行转换
   • json_string: 使用 json.dumps(val, ensure_ascii=False) 进行转换
   • boolean: 使用 bool(val) 进行转换
2. 赋值类型增加 null 选项，允许将变量赋值为空
3. 截图：
   1. 类型转换截图：
      
      
   2. 赋值为 null 截图：
      
      

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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