[RFC]feat: enhance transformer plugin by adding a set-based requirement check

[rayne-Li]

Ⅰ. Describe what this PR did

transformer插件目前有丰富的转换逻辑, 转换相关的功能符合我的使用场景, 但是目前我希望在add header:A时, 可以判断header中其他的key:B是否存在再进行添加(而非key:A本身)

因此我实现了一个基于sets的类似k8s labelSelector风格的校验机制, 它实现了例如: 当model不存在在headers中时才添加routing-strategy这个header

reqRules:
- headers:
  - key: "routing-strategy"
    requirement:
      key: "model"
      operator: "DoesNotExist" // Exists DoesNotExist In NotIn
    value: "random"
  operate: "add"

目前代码逻辑仅对kvHandler的add功能生效, 理论上任何一个operator都可以加上类似的条件判断机制, 如果社区认为这种方式是一个合理的必要的拓展, 我乐意继续完善add之外的功能

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Ⅰ. Describe what this PR did

The transformer plug-in currently has rich conversion logic, and the conversion-related functions are in line with my usage scenarios, but currently I hope that when adding header:A, I can determine whether other key:B in the header exists before adding it (instead of key:A itself)

Therefore, I implemented a verification mechanism based on sets similar to k8s labelSelector style. It implements, for example: only add the routing-strategy header when the model does not exist in the headers.

reqRules:
- headers:
  - key: "routing-strategy"
    requirement:
      key: "model"
      operator: "DoesNotExist" // Exists DoesNotExist In NotIn
    value: "random"
  operate: "add"

At present, the code logic only takes effect for the add function of kvHandler. In theory, any operator can add a similar conditional judgment mechanism. If the community believes that this method is a reasonable and necessary expansion, I will be happy to continue to improve functions other than add.

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