[Kotlin][Client] create request config method

[4brunu]

Hi,
Currently I have a problem when that in some situations, I need to make a request outside of generated code by OpenAPI.

For example, if I have an endpoint that returns an image, and I want to load that image with an image loader, for example Glide or Picasso, that already handles the caching and scalling part, I need to manually configure the request.

Or if I need to make a network request to create a bearer token in the server API, I need to manually make the request.

In those situations, I need to manually create the url and configure the request.
This this leads to errors especially when updating to a news version of the API.

In order to solve this, I created one extra method that returns the RequestConfig which can be used to get the request info.

This was inspired in the Swift generator.

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@jimschubert (2017/09) ❤️, @dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03)

[4brunu]

Done 👍

[wing328]

@4brunu thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423