Open ValueAugmenter in ValueAugmenterFactory for extension

Opening up the class `ValueAugmenter` in the `ValueAugmenterFactory` to `public` access level eases the creation of custom document transformers.

Currently, we have to place our custom document transformers in the `org.apache.solr.response.transform` package to be able to access the package private `ValueAugmenter`. Changing the visibility to `public` enables building custom document transformers very slim and quick like the following one in any package:

```
public class CollectionAugmenterFactory extends TransformerFactory {

@Override
public DocTransformer create(String field, SolrParams params, SolrQueryRequest req)

{ String v = ComponentUtils.getCollectionNameFrom(req.getCore()).orElse("[unknown]"); return new ValueAugmenterFactory.ValueAugmenter(field, v); }

}
```

I'll supply a GitHub PR for this.