[Kotlin][Multiplatform] Support custom Ktor HTTP client configuration

Created by: tkirshboim

PR Description

The Problem

It is currently not possible to configure the Ktor HTTP client when using generated Kotlin Multiplatform code. Configuring the HTTP client can be useful for logging or customising request headers, to name a few use cases.

This issue has been raised in #10130 (closed) .

The Solution

This PR makes it possible to configure the Ktor HTTP client by allowing to pass an optional httpClientConfig parameter to the generated API classes.

An example of how the new httpClientConfig parameter can be used can be seen here:

private val petApi = PetApi(httpClientConfig = createHttpClientConfig())

private fun createHttpClientConfig(): ((HttpClientConfig<*>) -> Unit) =
        { clientConfig ->
            clientConfig.install(UserAgent) {
                agent = "my-pet-store-app/${Platform().platform}"
            }
            clientConfig.install(Logging) {
                logger = object : Logger {
                    override fun log(message: String) {
                        println("HttpClient: $message")
                    }
                }
                level = LogLevel.ALL
            }
        }

Fixes #10130 (closed)

CCs

Kotlin technical committee: @jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m

Reporter of #10130 (closed) : @krzema12

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.