Add `Parallel Assets Override` feature to Assets Stack Management

When you want override all assets of a scope, you can redefine all your assets in a JSON file (if you use AssetsJsonLoader) by use `override` parameter set to `true`.

This feature is useful for override some assets available in some libraries

Original Assets :

{
    "scope": "your_scope",
    "assets": [
        {
            "name": "asset",
            "version": "bugged_version",
            "type": "js",
            "locations": { ... }
        }
    ]
 }

 Override Assets :

 {
     "scope": "your_scope",
     "override": true,
     "assets": [
         {
             "name": "asset",
             "version": "good_version",
             "type": "js",
             "locations": { ... }
         }
     ]
  }