[Go] Add multiple servers support to Go(-experimental) client

Created by: jirikuncar

Allows the Go(-Experimental) API client to retrieve all server URLs defined in the spec

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cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

related PRs

• https://github.com/OpenAPITools/openapi-generator/pull/1964
• https://github.com/OpenAPITools/openapi-generator/pull/1969
• https://github.com/OpenAPITools/openapi-generator/pull/1280