have option to add namespace and own commenting to generated types

[maapteh]

Hi, i now do post processing with your generated file. But it would be nice to have a namespace option with this tooling itself. There are multiple routes to it, so im giving two examples:

• implementation like https://www.npmjs.com/package/dtsgenerator but with this as option, not a default.
• configuration for adding content (prefix, suffix), see one of the best auto generators for flexibility https://www.graphql-code-generator.com/docs/plugins/add#examples

With a prefix and suffix option i would be able to add my own comments, and wrap it all in a namespace :)

Rationale: i export directly into a .d.ts file and i just like to be able to use: ContentfullRaw.IComponentBanner with no need to import anything in my application. Also when not in a 'd.ts' file a namespace would still be nice!

/* eslint-disable */
/* AUTO GENERATED DO NOT TOUCH, see tooling/xx/xx */
declare namespace ContentfullRaw {
  export interface IComponentBanner {
    sys: {
      id: string;