Separate non-generated parts of client (such as REST/API client) into a package

I am going to use python as an example, but this can be applied to most of other client libraries too.

Right now, some part of client files such as (https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/python/api_client.mustache) is not affected by input. These files are mainly responsible for providing http/auth support for API calls.

I suggest we move them into a separate repo (swagger-base-clients) and publish them as an stand alone working client and then put a dependency on that package into generated client.

This further allow us (kubernetes) to develop our own base client with interface compatibility with swagger-base-client and switch it in kubernetes generated client.

If there is community interest in this, I can write a proposal/mock up for one language to see how it look like.

Start with modules/swagger-codegen/src/main/resources/python/api_client.mustache and compare its non-generated HTTP and authentication responsibilities with the generated client structure. The issue proposes a separate swagger-base-clients repository and a dependency, but leaves the language, package design, compatibility interface, and implementation scope open. A useful next step would be a proposal or mock-up for one language, with a defined standalone client and integration boundary.