feat: Variable assign 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，以及添加赋值类型 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，比较直观，不再是选择 引用变量 然后不选择引用来设置为空了。
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.

[f2c-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment