refactor: Extract LLM validate logic & add ZhiPuAI validator

[Lanture1064]

Towards #28

[bjwswang]

@Lanture1064 You can define a common interface in pkg/llms/llms.go like

type Validator interface {
   Validate(llmUrl string) error
}

Then implement this in different llms. Take zhipuai as an example

func (z *ZhiPuAI)Validate(url string) error

[bjwswang]

if the status code in reponse data is 200,error can be nil here. Otherwise return a error indicates why this failed.