proposed solution for #11785

This is a prototype of a server code generator for NestJS. The code probably needs improvement, I'm relatively new in this codebase and tried to adhere to existing generators. As of now, I only want to gather feedback on the solution the generated output provides, and whether it's worth continuing this approach.

Description a per the added README

OpenApi Generator typescript-nestjs-server

Usage: The generated output is intended to be its own module, that can be imported into your NestJS App Module. You do not need to change generated files, just import the module and implement the API

Example usage (with the openapi sample petstore.yaml):

1. Invoke openapi-generator

   openapi-generator-cli.jar generate -i petstore.yaml -g typescript-nestjs-server -o api-module/
   

2. implement the contracts from api-module/api
handlers/PetService.ts:

   import { Pet, ApiResponse } from "models";
   import { Observable } from "rxjs";
   import { PetApi } from "../api";
   import { Inject, Injectable } from "@nestjs/common";
   
   @Injectable()
   export class PetService implements PetApi {
     addPet(pet: Pet, request: Request): Pet | Promise<Pet> | Observable<Pet> {
       throw new Error("Method not implemented.");
     }
   
     deletePet(petId: number, apiKey: string, request: Request): void | Promise<void> | Observable<void> {
       throw new Error("Method not implemented.");
     }
   
   ...

3. Import the generated ApiModule with ApiModule.forRoot and provide a instance of ApiImplementations with a reference to your implementation
   app.module.ts

   import { Module } from "@nestjs/common";
   import { ApiModule, ApiImplementations } from "api-module";
   import { PetService } from "./handlers/PetService";
   import { UserService } from "./handlers/UserService";
   import { StoreService } from "./handlers/StoreService";
   
   const apiImplementations: ApiImplementations = {
     petApi: PetService,
     userApi: UserService,
     storeApi: StoreService,
   }
   
   @Module({
     imports: [
       ApiModule.forRoot(apiImplementations),
     ],
     controllers: [],
     providers: [],
   })
   export class AppModule {}

You now can regenerate the API module as often as you want without overriding your implementation.

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo (2019/11) @amakhrov @davidgamero @mkusaka @joscha