[kotlin-spring] ResponseEntity option

Created by: frecco75

This PR adds ResponseEntity option (inspired by the spring generator).

By default the generator (before this PR) generates code with 200 return code.

Given this schema

openapi: "3.0.1"
info:
  version: 1.0.0
  title: Users

paths:
  /users/{userId}:
    get:
      parameters:
        - in: path
          name: userId
          required: true
          schema:
            type: string
      responses:
        200:
          description: user found
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        404:
          description: user not found
  
components:
  schemas:
    User:
      type: object
      required:
        - name
      properties:
        name:
          type: string

It generates by default this code

fun usersUserIdGet(@ApiParam(value = "", required=true) @PathVariable("userId") userId: kotlin.String
): ResponseEntity<User> {
        return ResponseEntity(service.usersUserIdGet(userId), HttpStatus.valueOf(200))
    }

which doesn't respect the schema response code when user isn't found.

Providing the option responseEntity=true it generates

fun usersUserIdGet(@ApiParam(value = "", required=true) @PathVariable("userId") userId: kotlin.String
): ResponseEntity<User> {
        return service.usersUserIdGet(userId)

    }

and for the service

interface UsersApiService {
    fun usersUserIdGet(userId: kotlin.String): ResponseEntity<User>
}

so the developer can provide custom behaviour and provide status code, headers etc.. in the response