Add OpenAPI specs decorator

[gonzauy]

Adds an OpenAPI specs decorator.

Endpoints could add a new decoration and send data to be consumed by the analyze method.

On the app side, we can add a new wrapper like:

import { specs } from 'async-app';
import { Middleware, Req } from './app';

export type SpecsMetadata = {
  operationId?: string;
};

const specsMiddleware: Middleware = (_req: Req<null>, _res, next) => {
  next();
};

export const openApi = (spec: SpecsMetadata) => specs(spec, specsMiddleware);

And then use it like:

app.get(
  '/todos/:todoId',
  'Returns the specified todo item',
  openApi({ operationId: 'getTodoById' }),
  ...

When we call the analyze, we'll be able to access to the specs of each Route.