Support interface and implementation classes for API controllers

Description

For auto-generated server code, we want to generated 2 files for each API controller file:

1. an interface, which will be overwritten by code generation
2. an implementation class, which will not be overwritten by code generation

Java Spring has already implemented this:
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/JavaSpring/api.mustache
https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/resources/JavaSpring/apiController.mustache

The goal is to avoid application/business logic being overwritten by code generation so that there's less overhead when adding/deleting/updating endpoint definition.

Other server generators should leverage similar design.

Swagger-codegen version

Latest master

Suggest a Fix

If anyone has suggestions for better design or want to work on this enhancement for a particular server generator, please reply to let us know. Thank you.

Start by reviewing the referenced JavaSpring templates, api.mustache and apiController.mustache, to understand how interface and implementation files are separated. Then identify which server generator should be addressed first and inspect its corresponding templates and tests. Done means that generator emits both files while preserving implementation code across regeneration; this issue names no target generator or test.