[Go] Support streaming responses in go client

[roycaihw]

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  @wing328 @antihax

Description of the PR

Like python client #4166, we want Go API client to not read response content and return the http.Response with Body not closed, to support streaming responses. This PR add a separate method for each operation to keep current behavior.

Alternatives considered:

1. Add a stream boolean parameter to current method: this would break current behavior, and the methods still have returnType in function signature, which is not flexible (e.g. you cannot use different operation methods as same parameter)
2. Break the current method into two steps: func (a *SomeApiService) GetFooRawResp(...) (FooResponseStruct) and func (f *FooResponseStruct) DecodeFooResponse() (Foo, *http.Response, error), where FooResponseStruct contains the raw http response and error. The downside is same as the first one
3. Break the current method into two methods of (a *SomeApiService): func (a *SomeApiService) GetFooRawResp(...) (*http.Response, error) and func (f *FooResponseStruct) DecodeFooResponse(*http.Response) (Foo, *http.Response, error). Flexible, but breaks current behavior.