akka-http

This is a part of #11

I decided to extract it since it is a big feature.

Hi there, thanks for this project. Very happy so far.

We're trying to migrate to scala 3, and on of the things which came up was this.

I need to make this manual change to use a generated http4s client, in Implicits.scala

-    implicit val showString = build[String](Predef.identity)
-    implicit val showInt = build[Int](_.toString)
-    implicit val showLong = build[Long](_.

We make sure in the CI configuration to build with jdk8, but it would be safer to specify this in the build (javacOptions/scalacOptions) explicitly.

Add special directive allowing to automatically apply CORS rules according to implicit config

https://github.com/scalameta/mdoc