[Scala] Consolidate `scala`, `async-scala`, `akka-scala` into a single Scala API client

Description

Similar to Java, C#, Python API clients, we want to be able to provide a single Scala API client to perform synchronous and asynchronous HTTP calls.

Swagger-codegen version

Latest master

If anyone from the community wants to work on the consolidation, please refer to let us know.

A good starting point is to review how Java API client supports both synchronous and asynchronous calls and implement the same logic in the scala or async-scala generators and templates.