review incubator-shenyu-client-golang #3 pr

[Lisandro]

Dear community:

 The  incubator-shenyu-client-golang   has new  pr (add shenyu_admin_client & 
client_factory])，please review the code.Here is this pr main function:1.Add 
README
2.Abstract extra param
3.Add client_facory.go、shenyu_admin_client.go
4Refactor static param Here is link   
https://github.com/apache/incubator-shenyu-client-golang/pull/3/files