review incubator-shenyu-client-golang #4 pr

[Lisandro]

Dear community:

 The  incubator-shenyu-client-golang   has new  pr,please review the code.Here 
is this pr main function:1.Add README how to use shenyu admin client.
2.Add admin_client example  model.
3.Refactor shenyu_admin_client. Here is link   
https://github.com/apache/incubator-shenyu-client-golang/pull/4/files