Update AssetBundle.php

Added `assetKeys` property - custom keys to be attached for asset files in bundle config to be passed later 
to [[\yii\web\View::registerJsFile()]] or [[\yii\web\View::registerCssFile()]] if you don't want
default keys auto-assignment (this may be useful if you have 2 different extensions that load the same
js script on one page).

Then it could be set in config file like this:
```php
// ...
'components' => [
  'assetManager' => [
      'bundles' => [
          'foo\abc\SomeAsset' => [
               'assetKeys' => ['somefile.js' => 'somekey']
          ],
          'bar\cde\AnotherAsset' => [
               'assetKeys' => ['somefile.js' => 'somekey']
          ],
      ],
  ],
],
```