Custom generated spec behavior configuration

[AntoineABI]

Allow to alter the way custom spec (from tsao.json by default) is merged to generated swagger spec by a configuration option in SwaggerConfig interface named specMerging. The default value is meant to not break existing projects meaning this update is harmless.

 /**
   * Alter how the spec is merged to generated swagger spec.
   * Possible values:
   *  - 'immediate' is overriding top level elements only thus you can not append a new path or alter an existing value without erasing same level elements.
   *  - 'recursive' proceed to a deep merge and will concat every branches or override or create new values if needed. @see https://www.npmjs.com/package/merge
   * The default is set to immediate so it is not breaking previous versions.
   * @default 'immediate'
   */
  specMerging?: 'immediate' | 'recursive';

[lukeautry]

Nice @AntoineABI , I really like this approach.