SolrJ: Support Converter and make it easier to extend DocumentObjectBinder

In our old project, we use Spring-Solr, it provides some good function such as allow us to define converters to serialize java enum to solr string and vice verse, serialize object as json string and vice verse.

But it doesn't support latest solr, solr cloud and child documents.

We would like to use pure solrj, but we do like spring solr's converters function.

Is it possible that SolrJ can support custom convert in SolrJ?

Also SolrJ should make it easier to extend DocumentObjectBinder, such as make DocField, infocache, getDocFields etc accessible in sub class.