[REQ] Kotlin ApiClient creates its own HttpClient instance, this is not well suited if used on the server

Is your feature request related to a problem? Please describe.

When using the generated client on the server, creating HttpClient on every request is expensive, ideally we would like to inject the instance of HttpClient ourselves and keep the ApiClient simple.

Describe the solution you'd like

A template that allows for injection of HttpClient would be ideal

Describe alternatives you've considered

There is a way to pass in the engine (OkHttp) for example, but this solves only part of the problem

@jimschubert
@andrewemery
@alisters

Start by locating the Kotlin generated ApiClient template and examining how it currently creates HttpClient instances and accepts an engine. Define the injection approach for a caller-provided HttpClient, then verify that generated clients can reuse that instance without creating one per request.