fix(contrib/registry/eureka): use shared http.Transport

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Use a shared http.Transport to enable connection reuse and prevent connection accumulation.
Previously, each NewClient() call create a new http.Transport, which led to:

• Independent connection pools per client instance
• Inability to reuse connections across clients
• Connection accumulation due to keep-alive connection not being shared

In theory, NewClient should only be called once. However, users may call it multiple.
Meanwhile, to follow Go best practices, we should modify the code to reuse http.Transport.
Ref: https://github.com/golang/go/blob/master/src/net/http/client.go#L33

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Also: https://juejin.cn/post/7372404987702378536